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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
        Will Deacon <will.deacon@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.9 028/310] perf/callchain: Force USER_DS when invoking perf_callchain_user()
Date: Wed, 11 Apr 2018 20:32:47 +0200
Message-Id: <20180411183623.502566763@linuxfoundation.org>
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4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/events/callchain.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -227,12 +227,18 @@ get_perf_callchain(struct pt_regs *regs,
 		}
 
 		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);
 		}
 	}