linux-hardening - [PATCH] mm: Fix vm_brk_flags() to not bail out while holding lock

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Date: Fri, 29 Sep 2023 10:19:41 -0700
From: Kees Cook <keescook@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>,
	Sebastian Ott <sebott@...hat.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Yu Zhao <yuzhao@...gle.com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] mm: Fix vm_brk_flags() to not bail out while holding lock

From: Sebastian Ott <sebott@...hat.com>

Calling vm_brk_flags() with flags set other than VM_EXEC
will exit the function without releasing the mmap_write_lock.

Just do the sanity check before the lock is acquired. This
doesn't fix an actual issue since no caller sets a flag other
than VM_EXEC.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Liam R. Howlett <Liam.Howlett@...cle.com>
Cc: Yu Zhao <yuzhao@...gle.com>
Cc: linux-mm@...ck.org
Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()")
Signed-off-by: Sebastian Ott <sebott@...hat.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 mm/mmap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 34d2337ace59..c8996fe847c9 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3143,13 +3143,13 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
 	if (!len)
 		return 0;
 
-	if (mmap_write_lock_killable(mm))
-		return -EINTR;
-
 	/* Until we need other flags, refuse anything except VM_EXEC. */
 	if ((flags & (~VM_EXEC)) != 0)
 		return -EINVAL;
 
+	if (mmap_write_lock_killable(mm))
+		return -EINTR;
+
 	ret = check_brk_limits(addr, len);
 	if (ret)
 		goto limits_failed;
-- 
2.34.1