[PATCH v2] loop: check for read-write open mode on LOOP_SET_STATUS in lo_compat_ioctl()

[Dominik Maier <dmnk@google.com>]

lo_ioctl() checks for (mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN)
before allowing LOOP_SET_STATUS or LOOP_SET_STATUS64, but not
lo_compat_ioctl(), the handler used by 32-bit user space on 64-bit
kernels.

As a result, on 64-bit kernels with CONFIG_COMPAT=y, a 32-bit
unprivileged user-space process holding a read-only (O_RDONLY) file
descriptor to an active /dev/loopN device can issue a compat
LOOP_SET_STATUS ioctl to modify lo_offset, clear lo_sizelimit to 0
(exposing backing-file data past the configured limit), or alter
lo_flags.

Enforce the same (mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN) check
on LOOP_SET_STATUS in lo_compat_ioctl().

Assisted-by: ARTIST LLM
Signed-off-by: Dominik Maier <dmnk@google.com>
---
v2:
  - Rephrase commit description of lo_compat_ioctl() and remove
    inaccurate Fixes tag per Bart Van Assche.

 drivers/block/loop.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 758c20678bf6..f15a5cae5c63 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1708,8 +1708,10 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
 
 	switch(cmd) {
 	case LOOP_SET_STATUS:
-		err = loop_set_status_compat(lo,
-			     (const struct compat_loop_info __user *)arg);
+		err = -EPERM;
+		if ((mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN))
+			err = loop_set_status_compat(lo,
+				(const struct compat_loop_info __user *)arg);
 		break;
 	case LOOP_GET_STATUS:
 		err = loop_get_status_compat(lo,
-- 
2.56.0.257.gf7497c7d27-goog