linux/kernel/trace/bpf_trace.c, branch v5.14

bpf: Add lockdown check for probe_write_user helper

2021-08-10T08:10:10Z

Back then, commit 96ae52279594 ("bpf: Add bpf_probe_write_user BPF helper to be called in tracers") added the bpf_probe_write_user() helper in order to allow to override user space memory. Its original goal was to have a facility to "debug, divert, and manipulate execution of semi-cooperative processes" under CAP_SYS_ADMIN. Write to kernel was explicitly disallowed since it would otherwise tamper with its integrity. One use case was shown in cf9b1199de27 ("samples/bpf: Add test/example of using bpf_probe_write_user bpf helper") where the program DNATs traffic at the time of connect(2) syscall, meaning, it rewrites the arguments to a syscall while they're still in userspace, and before the syscall has a chance to copy the argument into kernel space. These days we have better mechanisms in BPF for achieving the same (e.g. for load-balancers), but without having to write to userspace memory. Of course the bpf_probe_write_user() helper can also be used to abuse many other things for both good or bad purpose. Outside of BPF, there is a similar mechanism for ptrace(2) such as PTRACE_PEEK{TEXT,DATA} and PTRACE_POKE{TEXT,DATA}, but would likely require some more effort. Commit 96ae52279594 explicitly dedicated the helper for experimentation purpose only. Thus, move the helper's availability behind a newly added LOCKDOWN_BPF_WRITE_USER lockdown knob so that the helper is disabled under the "integrity" mode. More fine-grained control can be implemented also from LSM side with this change. Fixes: 96ae52279594 ("bpf: Add bpf_probe_write_user BPF helper to be called in tracers") Signed-off-by: Daniel Borkmann Acked-by: Andrii Nakryiko

bpf: Add _kernel suffix to internal lockdown_bpf_read

2021-08-09T19:50:41Z

Rename LOCKDOWN_BPF_READ into LOCKDOWN_BPF_READ_KERNEL so we have naming more consistent with a LOCKDOWN_BPF_WRITE_USER option that we are adding. Signed-off-by: Daniel Borkmann Acked-by: Andrii Nakryiko

Merge tag 'trace-v5.14' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

2021-07-03T18:13:22Z

Pull tracing updates from Steven Rostedt: - Added option for per CPU threads to the hwlat tracer - Have hwlat tracer handle hotplug CPUs - New tracer: osnoise, that detects latency caused by interrupts, softirqs and scheduling of other tasks. - Added timerlat tracer that creates a thread and measures in detail what sources of latency it has for wake ups. - Removed the "success" field of the sched_wakeup trace event. This has been hardcoded as "1" since 2015, no tooling should be looking at it now. If one exists, we can revert this commit, fix that tool and try to remove it again in the future. - tgid mapping fixed to handle more than PID_MAX_DEFAULT pids/tgids. - New boot command line option "tp_printk_stop", as tp_printk causes trace events to write to console. When user space starts, this can easily live lock the system. Having a boot option to stop just after boot up is useful to prevent that from happening. - Have ftrace_dump_on_oops boot command line option take numbers that match the numbers shown in /proc/sys/kernel/ftrace_dump_on_oops. - Bootconfig clean ups, fixes and enhancements. - New ktest script that tests bootconfig options. - Add tracepoint_probe_register_may_exist() to register a tracepoint without triggering a WARN*() if it already exists. BPF has a path from user space that can do this. All other paths are considered a bug. - Small clean ups and fixes * tag 'trace-v5.14' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace: (49 commits) tracing: Resize tgid_map to pid_max, not PID_MAX_DEFAULT tracing: Simplify & fix saved_tgids logic treewide: Add missing semicolons to __assign_str uses tracing: Change variable type as bool for clean-up trace/timerlat: Fix indentation on timerlat_main() trace/osnoise: Make 'noise' variable s64 in run_osnoise() tracepoint: Add tracepoint_probe_register_may_exist() for BPF tracing tracing: Fix spelling in osnoise tracer "interferences" -> "interference" Documentation: Fix a typo on trace/osnoise-tracer trace/osnoise: Fix return value on osnoise_init_hotplug_support trace/osnoise: Make interval u64 on osnoise_main trace/osnoise: Fix 'no previous prototype' warnings tracing: Have osnoise_main() add a quiescent state for task rcu seq_buf: Make trace_seq_putmem_hex() support data longer than 8 seq_buf: Fix overflow in seq_buf_putmem_hex() trace/osnoise: Support hotplug operations trace/hwlat: Support hotplug operations trace/hwlat: Protect kdata->kthread with get/put_online_cpus trace: Add timerlat tracer trace: Add osnoise tracer ...

tracepoint: Add tracepoint_probe_register_may_exist() for BPF tracing

2021-06-29T15:51:25Z

All internal use cases for tracepoint_probe_register() is set to not ever be called with the same function and data. If it is, it is considered a bug, as that means the accounting of handling tracepoints is corrupted. If the function and data for a tracepoint is already registered when tracepoint_probe_register() is called, it will call WARN_ON_ONCE() and return with EEXISTS. The BPF system call can end up calling tracepoint_probe_register() with the same data, which now means that this can trigger the warning because of a user space process. As WARN_ON_ONCE() should not be called because user space called a system call with bad data, there needs to be a way to register a tracepoint without triggering a warning. Enter tracepoint_probe_register_may_exist(), which can be called, but will not cause a WARN_ON() if the probe already exists. It will still error out with EEXIST, which will then be sent to the user space that performed the BPF system call. This keeps the previous testing for issues with other users of the tracepoint code, while letting BPF call it with duplicated data and not warn about it. Link: https://lore.kernel.org/lkml/20210626135845.4080-1-penguin-kernel@I-love.SAKURA.ne.jp/ Link: https://syzkaller.appspot.com/bug?id=41f4318cf01762389f4d1c1c459da4f542fe5153 Cc: stable@vger.kernel.org Fixes: c4f6699dfcb85 ("bpf: introduce BPF_RAW_TRACEPOINT") Reported-by: syzbot Reported-by: Tetsuo Handa Tested-by: syzbot+721aa903751db87aa244@syzkaller.appspotmail.com Signed-off-by: Steven Rostedt (VMware)

bpf: Allow bpf_get_current_ancestor_cgroup_id for tracing

2021-06-28T13:43:02Z

Allow the helper to be called from tracing programs. This is needed to handle cgroup hiererachies in the program. Signed-off-by: Namhyung Kim Signed-off-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20210627153627.824198-1-namhyung@kernel.org

bpf, lockdown, audit: Fix buggy SELinux lockdown permission checks

2021-06-02T19:59:22Z

Commit 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown") added an implementation of the locked_down LSM hook to SELinux, with the aim to restrict which domains are allowed to perform operations that would breach lockdown. This is indirectly also getting audit subsystem involved to report events. The latter is problematic, as reported by Ondrej and Serhei, since it can bring down the whole system via audit: 1) The audit events that are triggered due to calls to security_locked_down() can OOM kill a machine, see below details [0]. 2) It also seems to be causing a deadlock via avc_has_perm()/slow_avc_audit() when trying to wake up kauditd, for example, when using trace_sched_switch() tracepoint, see details in [1]. Triggering this was not via some hypothetical corner case, but with existing tools like runqlat & runqslower from bcc, for example, which make use of this tracepoint. Rough call sequence goes like: rq_lock(rq) -> -------------------------+ trace_sched_switch() -> | bpf_prog_xyz() -> +-> deadlock selinux_lockdown() -> | audit_log_end() -> | wake_up_interruptible() -> | try_to_wake_up() -> | rq_lock(rq) --------------+ What's worse is that the intention of 59438b46471a to further restrict lockdown settings for specific applications in respect to the global lockdown policy is completely broken for BPF. The SELinux policy rule for the current lockdown check looks something like this: allow : lockdown { }; However, this doesn't match with the 'current' task where the security_locked_down() is executed, example: httpd does a syscall. There is a tracing program attached to the syscall which triggers a BPF program to run, which ends up doing a bpf_probe_read_kernel{,_str}() helper call. The selinux_lockdown() hook does the permission check against 'current', that is, httpd in this example. httpd has literally zero relation to this tracing program, and it would be nonsensical having to write an SELinux policy rule against httpd to let the tracing helper pass. The policy in this case needs to be against the entity that is installing the BPF program. For example, if bpftrace would generate a histogram of syscall counts by user space application: bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @[comm] = count(); }' bpftrace would then go and generate a BPF program from this internally. One way of doing it [for the sake of the example] could be to call bpf_get_current_task() helper and then access current->comm via one of bpf_probe_read_kernel{,_str}() helpers. So the program itself has nothing to do with httpd or any other random app doing a syscall here. The BPF program _explicitly initiated_ the lockdown check. The allow/deny policy belongs in the context of bpftrace: meaning, you want to grant bpftrace access to use these helpers, but other tracers on the system like my_random_tracer _not_. Therefore fix all three issues at the same time by taking a completely different approach for the security_locked_down() hook, that is, move the check into the program verification phase where we actually retrieve the BPF func proto. This also reliably gets the task (current) that is trying to install the BPF tracing program, e.g. bpftrace/bcc/perf/systemtap/etc, and it also fixes the OOM since we're moving this out of the BPF helper's fast-path which can be called several millions of times per second. The check is then also in line with other security_locked_down() hooks in the system where the enforcement is performed at open/load time, for example, open_kcore() for /proc/kcore access or module_sig_check() for module signatures just to pick few random ones. What's out of scope in the fix as well as in other security_locked_down() hook locations /outside/ of BPF subsystem is that if the lockdown policy changes on the fly there is no retrospective action. This requires a different discussion, potentially complex infrastructure, and it's also not clear whether this can be solved generically. Either way, it is out of scope for a suitable stable fix which this one is targeting. Note that the breakage is specifically on 59438b46471a where it started to rely on 'current' as UAPI behavior, and _not_ earlier infrastructure such as 9d1f8be5cf42 ("bpf: Restrict bpf when kernel lockdown is in confidentiality mode"). [0] https://bugzilla.redhat.com/show_bug.cgi?id=1955585, Jakub Hrozek says: I starting seeing this with F-34. When I run a container that is traced with BPF to record the syscalls it is doing, auditd is flooded with messages like: type=AVC msg=audit(1619784520.593:282387): avc: denied { confidentiality } for pid=476 comm="auditd" lockdown_reason="use of bpf to read kernel RAM" scontext=system_u:system_r:auditd_t:s0 tcontext=system_u:system_r:auditd_t:s0 tclass=lockdown permissive=0 This seems to be leading to auditd running out of space in the backlog buffer and eventually OOMs the machine. [...] auditd running at 99% CPU presumably processing all the messages, eventually I get: Apr 30 12:20:42 fedora kernel: audit: backlog limit exceeded Apr 30 12:20:42 fedora kernel: audit: backlog limit exceeded Apr 30 12:20:42 fedora kernel: audit: audit_backlog=2152579 > audit_backlog_limit=64 Apr 30 12:20:42 fedora kernel: audit: audit_backlog=2152626 > audit_backlog_limit=64 Apr 30 12:20:42 fedora kernel: audit: audit_backlog=2152694 > audit_backlog_limit=64 Apr 30 12:20:42 fedora kernel: audit: audit_lost=6878426 audit_rate_limit=0 audit_backlog_limit=64 Apr 30 12:20:45 fedora kernel: oci-seccomp-bpf invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0, oom_score_adj=-1000 Apr 30 12:20:45 fedora kernel: CPU: 0 PID: 13284 Comm: oci-seccomp-bpf Not tainted 5.11.12-300.fc34.x86_64 #1 Apr 30 12:20:45 fedora kernel: Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014 [...] [1] https://lore.kernel.org/linux-audit/CANYvDQN7H5tVp47fbYcRasv4XF07eUbsDwT_eDCHXJUj43J7jQ@mail.gmail.com/, Serhei Makarov says: Upstream kernel 5.11.0-rc7 and later was found to deadlock during a bpf_probe_read_compat() call within a sched_switch tracepoint. The problem is reproducible with the reg_alloc3 testcase from SystemTap's BPF backend testsuite on x86_64 as well as the runqlat, runqslower tools from bcc on ppc64le. Example stack trace: [...] [ 730.868702] stack backtrace: [ 730.869590] CPU: 1 PID: 701 Comm: in:imjournal Not tainted, 5.12.0-0.rc2.20210309git144c79ef3353.166.fc35.x86_64 #1 [ 730.871605] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014 [ 730.873278] Call Trace: [ 730.873770] dump_stack+0x7f/0xa1 [ 730.874433] check_noncircular+0xdf/0x100 [ 730.875232] __lock_acquire+0x1202/0x1e10 [ 730.876031] ? __lock_acquire+0xfc0/0x1e10 [ 730.876844] lock_acquire+0xc2/0x3a0 [ 730.877551] ? __wake_up_common_lock+0x52/0x90 [ 730.878434] ? lock_acquire+0xc2/0x3a0 [ 730.879186] ? lock_is_held_type+0xa7/0x120 [ 730.880044] ? skb_queue_tail+0x1b/0x50 [ 730.880800] _raw_spin_lock_irqsave+0x4d/0x90 [ 730.881656] ? __wake_up_common_lock+0x52/0x90 [ 730.882532] __wake_up_common_lock+0x52/0x90 [ 730.883375] audit_log_end+0x5b/0x100 [ 730.884104] slow_avc_audit+0x69/0x90 [ 730.884836] avc_has_perm+0x8b/0xb0 [ 730.885532] selinux_lockdown+0xa5/0xd0 [ 730.886297] security_locked_down+0x20/0x40 [ 730.887133] bpf_probe_read_compat+0x66/0xd0 [ 730.887983] bpf_prog_250599c5469ac7b5+0x10f/0x820 [ 730.888917] trace_call_bpf+0xe9/0x240 [ 730.889672] perf_trace_run_bpf_submit+0x4d/0xc0 [ 730.890579] perf_trace_sched_switch+0x142/0x180 [ 730.891485] ? __schedule+0x6d8/0xb20 [ 730.892209] __schedule+0x6d8/0xb20 [ 730.892899] schedule+0x5b/0xc0 [ 730.893522] exit_to_user_mode_prepare+0x11d/0x240 [ 730.894457] syscall_exit_to_user_mode+0x27/0x70 [ 730.895361] entry_SYSCALL_64_after_hwframe+0x44/0xae [...] Fixes: 59438b46471a ("security,lockdown,selinux: implement SELinux lockdown") Reported-by: Ondrej Mosnacek Reported-by: Jakub Hrozek Reported-by: Serhei Makarov Reported-by: Jiri Olsa Signed-off-by: Daniel Borkmann Acked-by: Alexei Starovoitov Tested-by: Jiri Olsa Cc: Paul Moore Cc: James Morris Cc: Jerome Marchand Cc: Frank Eigler Cc: Linus Torvalds Link: https://lore.kernel.org/bpf/01135120-8bf7-df2e-cff0-1d73f1f841c3@iogearbox.net

bpf: Implement formatted output helpers with bstr_printf

2021-04-27T22:56:31Z

BPF has three formatted output helpers: bpf_trace_printk, bpf_seq_printf and bpf_snprintf. Their signatures specify that all arguments are provided from the BPF world as u64s (in an array or as registers). All of these helpers are currently implemented by calling functions such as snprintf() whose signatures take a variable number of arguments, then placed in a va_list by the compiler to call vsnprintf(). "d9c9e4db bpf: Factorize bpf_trace_printk and bpf_seq_printf" introduced a bpf_printf_prepare function that fills an array of u64 sanitized arguments with an array of "modifiers" which indicate what the "real" size of each argument should be (given by the format specifier). The BPF_CAST_FMT_ARG macro consumes these arrays and casts each argument to its real size. However, the C promotion rules implicitely cast them all back to u64s. Therefore, the arguments given to snprintf are u64s and the va_list constructed by the compiler will use 64 bits for each argument. On 64 bit machines, this happens to work well because 32 bit arguments in va_lists need to occupy 64 bits anyway, but on 32 bit architectures this breaks the layout of the va_list expected by the called function and mangles values. In "88a5c690b6 bpf: fix bpf_trace_printk on 32 bit archs", this problem had been solved for bpf_trace_printk only with a "horrid workaround" that emitted multiple calls to trace_printk where each call had different argument types and generated different va_list layouts. One of the call would be dynamically chosen at runtime. This was ok with the 3 arguments that bpf_trace_printk takes but bpf_seq_printf and bpf_snprintf accept up to 12 arguments. Because this approach scales code exponentially, it is not a viable option anymore. Because the promotion rules are part of the language and because the construction of a va_list is an arch-specific ABI, it's best to just avoid variadic arguments and va_lists altogether. Thankfully the kernel's snprintf() has an alternative in the form of bstr_printf() that accepts arguments in a "binary buffer representation". These binary buffers are currently created by vbin_printf and used in the tracing subsystem to split the cost of printing into two parts: a fast one that only dereferences and remembers values, and a slower one, called later, that does the pretty-printing. This patch refactors bpf_printf_prepare to construct binary buffers of arguments consumable by bstr_printf() instead of arrays of arguments and modifiers. This gets rid of BPF_CAST_FMT_ARG and greatly simplifies the bpf_printf_prepare usage but there are a few gotchas that change how bpf_printf_prepare needs to do things. Currently, bpf_printf_prepare uses a per cpu temporary buffer as a generic storage for strings and IP addresses. With this refactoring, the temporary buffers now holds all the arguments in a structured binary format. To comply with the format expected by bstr_printf, certain format specifiers also need to be pre-formatted: %pB and %pi6/%pi4/%pI4/%pI6. Because vsnprintf subroutines for these specifiers are hard to expose, we pre-format these arguments with calls to snprintf(). Reported-by: Rasmus Villemoes Signed-off-by: Florent Revest Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/20210427174313.860948-3-revest@chromium.org

bpf: Lock bpf_trace_printk's tmp buf before it is written to

2021-04-27T15:04:34Z

bpf_trace_printk uses a shared static buffer to hold strings before they are printed. A recent refactoring moved the locking of that buffer after it gets filled by mistake. Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf") Reported-by: Rasmus Villemoes Signed-off-by: Florent Revest Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/20210427112958.773132-1-revest@chromium.org

bpf: Add a bpf_snprintf helper

2021-04-19T22:27:36Z

The implementation takes inspiration from the existing bpf_trace_printk helper but there are a few differences: To allow for a large number of format-specifiers, parameters are provided in an array, like in bpf_seq_printf. Because the output string takes two arguments and the array of parameters also takes two arguments, the format string needs to fit in one argument. Thankfully, ARG_PTR_TO_CONST_STR is guaranteed to point to a zero-terminated read-only map so we don't need a format string length arg. Because the format-string is known at verification time, we also do a first pass of format string validation in the verifier logic. This makes debugging easier. Signed-off-by: Florent Revest Signed-off-by: Alexei Starovoitov Acked-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20210419155243.1632274-4-revest@chromium.org

bpf: Factorize bpf_trace_printk and bpf_seq_printf

2021-04-19T22:27:36Z

Two helpers (trace_printk and seq_printf) have very similar implementations of format string parsing and a third one is coming (snprintf). To avoid code duplication and make the code easier to maintain, this moves the operations associated with format string parsing (validation and argument sanitization) into one generic function. The implementation of the two existing helpers already drifted quite a bit so unifying them entailed a lot of changes: - bpf_trace_printk always expected fmt[fmt_size] to be the terminating NULL character, this is no longer true, the first 0 is terminating. - bpf_trace_printk now supports %% (which produces the percentage char). - bpf_trace_printk now skips width formating fields. - bpf_trace_printk now supports the X modifier (capital hexadecimal). - bpf_trace_printk now supports %pK, %px, %pB, %pi4, %pI4, %pi6 and %pI6 - argument casting on 32 bit has been simplified into one macro and using an enum instead of obscure int increments. - bpf_seq_printf now uses bpf_trace_copy_string instead of strncpy_from_kernel_nofault and handles the %pks %pus specifiers. - bpf_seq_printf now prints longs correctly on 32 bit architectures. - both were changed to use a global per-cpu tmp buffer instead of one stack buffer for trace_printk and 6 small buffers for seq_printf. - to avoid per-cpu buffer usage conflict, these helpers disable preemption while the per-cpu buffer is in use. - both helpers now support the %ps and %pS specifiers to print symbols. The implementation is also moved from bpf_trace.c to helpers.c because the upcoming bpf_snprintf helper will be made available to all BPF programs and will need it. Signed-off-by: Florent Revest Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/20210419155243.1632274-2-revest@chromium.org