[PATCH] memfd: deny writeable mappings when implying SEAL_WRITE

[Pratyush Yadav]

From: "Pratyush Yadav (Google)" <pratyush@xxxxxxxxxx>

When SEAL_EXEC is added, SEAL_WRITE is implied to make W^X. But the
implied seal is set after the check that makes sure the memfd can not
have any writable mappings. This means one can use SEAL_EXEC to apply
SEAL_WRITE while having writeable mappings.

This breaks the contract that SEAL_WRITE provides and can be used by an
attacker to pass a memfd that appears to be write sealed but can still
be modified arbitrarily.

Fix this by adding the implied seals before the call for
mapping_deny_writable() is done.

Fixes: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>
---
 mm/memfd.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memfd.c b/mm/memfd.c
index fb425f4e315f..abe13b291ddc 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -283,6 +283,12 @@ int memfd_add_seals(struct file *file, unsigned int seals)
 		goto unlock;
 	}
 
+	/*
+	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
+	 */
+	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
+		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
+
 	if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) {
 		error = mapping_deny_writable(file->f_mapping);
 		if (error)
@@ -295,12 +301,6 @@ int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
-	/*
-	 * SEAL_EXEC implies SEAL_WRITE, making W^X from the start.
-	 */
-	if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
-		seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
-
 	*file_seals |= seals;
 	error = 0;
 
-- 
2.54.0.545.g6539524ca2-goog