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    Subject[PATCH AUTOSEL for 4.9 028/281] perf/callchain: Force USER_DS when invoking perf_callchain_user()
    Date
    From: Will Deacon <will.deacon@arm.com>

    [ Upstream commit 88b0193d9418c00340e45e0a913a0813bc6c8c96 ]

    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>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    kernel/events/callchain.c | 6 ++++++
    1 file changed, 6 insertions(+)

    diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
    index e9fdb5203de5..411226b26bca 100644
    --- a/kernel/events/callchain.c
    +++ b/kernel/events/callchain.c
    @@ -227,12 +227,18 @@ 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);
    }
    }

    --
    2.14.1
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