Commit 3b041514 authored May 05, 2026 by Pratyush Yadav (Google) Committed by Andrew Morton May 21, 2026

memfd: deny writeable mappings when implying SEAL_WRITE

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.

Link: https://lore.kernel.org/20260505133922.797635-1-pratyush@kernel.org


Fixes: c4f75bc8 ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd")
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Jeff Xu <jeffxu@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent fa0b9b2b

mm/memfd.c

+6 −6

Original line number	Diff line number	Diff line
		@@ -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;