[tip:core/urgent] stacktrace: Force USER_DS for stack_trace_save_user()

[tip-bot for Peter Zijlstra]

Commit-ID:  cac9b9a4b08304f11daace03b8b48659355e44c1
Gitweb:     https://git.kernel.org/tip/cac9b9a4b08304f11daace03b8b48659355e44c1
Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Thu, 18 Jul 2019 10:47:47 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Thu, 18 Jul 2019 16:47:24 +0200

stacktrace: Force USER_DS for stack_trace_save_user()

When walking userspace stacks, USER_DS needs to be set, otherwise
access_ok() will not function as expected.

Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Reported-by: Eiichi Tsukata <devel@xxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190718085754.GM3402@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/stacktrace.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
index e6a02b274b73..f5440abb7532 100644
--- a/kernel/stacktrace.c
+++ b/kernel/stacktrace.c
@@ -226,12 +226,17 @@ unsigned int stack_trace_save_user(unsigned long *store, unsigned int size)
 		.store	= store,
 		.size	= size,
 	};
+	mm_segment_t fs;
 
 	/* Trace user stack if not a kernel thread */
 	if (current->flags & PF_KTHREAD)
 		return 0;
 
+	fs = get_fs();
+	set_fs(USER_DS);
 	arch_stack_walk_user(consume_entry, &c, task_pt_regs(current));
+	set_fs(fs);
+
 	return c.len;
 }
 #endif