On Mon, Feb 25, 2019 at 09:33:09AM +0100, Peter Zijlstra wrote:
On Sun, Feb 24, 2019 at 09:26:45AM -0800, Linus Torvalds wrote:
PeterZ, do you remember the particular use case that triggered that commit 7c4788950ba5 ("x86/uaccess, sched/preempt: Verify access_ok() context")?
This one, if I'm not mistaken.
commit ae31fe51a3cceaa0cabdb3058f69669ecb47f12e Author: Johannes Weiner hannes@cmpxchg.org Date: Tue Nov 22 10:57:42 2016 +0100
perf/x86: Restore TASK_SIZE check on frame pointerThe following commit: 75925e1ad7f5 ("perf/x86: Optimize stack walk user accesses") ... switched from copy_from_user_nmi() to __copy_from_user_nmi() with a manual access_ok() check. Unfortunately, copy_from_user_nmi() does an explicit check against TASK_SIZE, whereas the access_ok() uses whatever the current address limit of the task is. We are getting NMIs when __probe_kernel_read() has switched to KERNEL_DS, and then see vmalloc faults when we access what looks like pointers into vmalloc space:
Also note that this was before we did:
commit 88b0193d9418c00340e45e0a913a0813bc6c8c96 Author: Will Deacon will.deacon@arm.com Date: Tue May 9 18:00:04 2017 +0100
perf/callchain: Force USER_DS when invoking perf_callchain_user()
Perf can generate and record a user callchain in response to a synchronous request, such as a tracepoint firing. If this happens under set_fs(KERNEL_DS), then we can end up walking the user stack (and dereferencing/saving whatever we find there) without the protections usually afforded by checks such as access_ok.
Rather than play whack-a-mole with each architecture's stack unwinding implementation, fix the root of the problem by ensuring that we force USER_DS when invoking perf_callchain_user from the perf core.
Reported-by: Al Viro viro@ZenIV.linux.org.uk Signed-off-by: Will Deacon will.deacon@arm.com Acked-by: Peter Zijlstra peterz@infradead.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Arnaldo Carvalho de Melo acme@redhat.com Cc: Jiri Olsa jolsa@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Ingo Molnar mingo@kernel.org
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c index c04917cad1bf..1b2be63c8528 100644 --- a/kernel/events/callchain.c +++ b/kernel/events/callchain.c @@ -229,12 +229,18 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user, }
if (regs) { + mm_segment_t fs; + if (crosstask) goto exit_put;
if (add_mark) perf_callchain_store_context(&ctx, PERF_CONTEXT_USER); + + fs = get_fs(); + set_fs(USER_DS); perf_callchain_user(&ctx, regs); + set_fs(fs); } }