From 3b041514cb6eae45869b020f743c14d983363222 Mon Sep 17 00:00:00 2001 From: "Pratyush Yadav (Google)" Date: Tue, 5 May 2026 15:39:20 +0200 Subject: [PATCH] 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: c4f75bc8bd6b ("mm/memfd: add write seals when apply SEAL_EXEC to executable memfd") Signed-off-by: Pratyush Yadav (Google) Reviewed-by: Pasha Tatashin Acked-by: Jeff Xu Cc: Baolin Wang Cc: Brendan Jackman Cc: Greg Thelen Cc: Hugh Dickins Cc: Kees Cook Cc: "David Hildenbrand (Arm)" Cc: Signed-off-by: Andrew Morton --- 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;