From b10054aa9d3a586f052a37e3eae3fc5199c5e8ea Mon Sep 17 00:00:00 2001
From: Dave Martin <Dave.Martin@arm.com>
Date: Tue, 31 Oct 2017 15:50:55 +0000
Subject: [PATCH] arm64: signal: Verify extra data is user-readable in
 sys_rt_sigreturn

Git-commit: abf73988a7c2b2130fbee7cd19d2f57695566050
Patch-mainline: v4.15-rc1
References: fate#325866

Currently sys_rt_sigreturn() verifies that the base sigframe is
readable, but no similar check is performed on the extra data to
which an extra_context record points.

This matters because the extra data will be read with the
unprotected user accessors.  However, this is not a problem at
present because the extra data base address is required to be
exactly at the end of the base sigframe.  So, there would need to
be a non-user-readable kernel address within about 59K
(SIGFRAME_MAXSZ - sizeof(struct rt_sigframe)) of some address for
which access_ok(VERIFY_READ) returns true, in order for sigreturn
to be able to read kernel memory that should be inaccessible to the
user task.  This is currently impossible due to the untranslatable
address hole between the TTBR0 and TTBR1 address ranges.

Disappearance of the hole between the TTBR0 and TTBR1 mapping
ranges would require the VA size for TTBR0 and TTBR1 to grow to at
least 55 bits, and either the disabling of tagged pointers for
userspace or enabling of tagged pointers for kernel space; none of
which is currently envisaged.

Even so, it is wrong to use the unprotected user accessors without
an accompanying access_ok() check.

To avoid the potential for future surprises, this patch does an
explicit access_ok() check on the extra data space when parsing an
extra_context record.

Fixes: 33f082614c34 ("arm64: signal: Allow expansion of the signal frame")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@suse.com>
---
 arch/arm64/kernel/signal.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 78ee2bbe4984..5c2f81c00c80 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -342,6 +342,10 @@ static int parse_user_sigframe(struct user_ctxs *user,
 			 */
 			offset = 0;
 			limit = extra_size;
+
+			if (!access_ok(VERIFY_READ, base, limit))
+				goto invalid;
+
 			continue;
 
 		default:
-- 
2.11.0