[PATCH v8 4/5] mm/memfd: Add write seals when apply SEAL_EXEC to executable memfd

[jeffxu@chromium.org]

From: Jeff Xu <jeffxu@google.com>

In order to avoid WX mappings, add F_SEAL_WRITE when apply
F_SEAL_EXEC to an executable memfd, so W^X from start.

This implys application need to fill the content of the memfd first,
after F_SEAL_EXEC is applied, application can no longer modify the
content of the memfd.

Typically, application seals the memfd right after writing to it.
For example:
1. memfd_create(MFD_EXEC).
2. write() code to the memfd.
3. fcntl(F_ADD_SEALS, F_SEAL_EXEC) to convert the memfd to W^X.
4. call exec() on the memfd.

Signed-off-by: Jeff Xu <jeffxu@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 mm/memfd.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/memfd.c b/mm/memfd.c
index ec70675a7069..92f0a5765f7c 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -222,6 +222,12 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
 		}
 	}
 
+	/*
+	 * SEAL_EXEC implys 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;
 
-- 
2.39.0.rc1.256.g54fd8350bd-goog