Commit 8f65ac0b authored by Isaac J. Manjarres's avatar Isaac J. Manjarres Committed by Andrew Morton
Browse files

mm/memfd: use strncpy_from_user() to read memfd name 

The existing logic uses strnlen_user() to calculate the length of the
memfd name from userspace and then copies the string into a buffer using
copy_from_user().  This is error-prone, as the string length could have
changed between the time when it was calculated and when the string was
copied.  The existing logic handles this by ensuring that the last byte in
the buffer is the terminating zero.

This handling is contrived and can better be handled by using
strncpy_from_user(), which gets the length of the string and copies it in
one shot.  Therefore, simplify the logic for copying the memfd name by
using strncpy_from_user().

No functional change.

Link: https://lkml.kernel.org/r/20250110165904.3437374-3-isaacmanjarres@google.com


Signed-off-by: default avatarIsaac J. Manjarres <isaacmanjarres@google.com>
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Reviewed-by: default avatarLorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: John Stultz <jstultz@google.com>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent f5dbcd90

mm/memfd.c

+6 −14
Original line number Diff line number Diff line
@@ -396,26 +396,18 @@ static char *alloc_name(const char __user *uname) 
	char *name;

	long len;



	/* length includes terminating zero */

	len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);

	if (len <= 0)

		return ERR_PTR(-EFAULT);

	if (len > MFD_NAME_MAX_LEN + 1)

		return ERR_PTR(-EINVAL);



	name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL);

	name = kmalloc(NAME_MAX + 1, GFP_KERNEL);

	if (!name)

		return ERR_PTR(-ENOMEM);



	strcpy(name, MFD_NAME_PREFIX);

	if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) {

	/* returned length does not include terminating zero */

	len = strncpy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, MFD_NAME_MAX_LEN + 1);

	if (len < 0) {

		error = -EFAULT;

		goto err_name;

	}



	/* terminating-zero may have changed after strnlen_user() returned */

	if (name[len + MFD_NAME_PREFIX_LEN - 1]) {

		error = -EFAULT;

	} else if (len > MFD_NAME_MAX_LEN) {

		error = -EINVAL;

		goto err_name;

	}