[PATCH] ufs: use u64 for directory size in ufs_last_byte

[YANXIN LI]

ufs1_read_inode() and ufs2_read_inode() copy the untrusted 64-bit
on-disk size into inode->i_size.  ufs_last_byte() then truncates that
value to an unsigned int before calculating the valid extent of a
directory folio.

A directory size of exactly 4 GiB is truncated to zero.  In
ufs_find_entry(), subtracting the requested record length from that zero
forms an endpoint almost 4 GiB beyond the mapped folio.  A crafted UFS2
image produces:

  BUG: KASAN: use-after-free in ufs_find_entry+0x583/0x6e0 [ufs]
  Read of size 1
  ...
  ufs_find_entry
  ufs_inode_by_name
  ufs_lookup
  __lookup_slow
  path_lookupat

The KASAN classification reflects that the adjacent physical page was
free; the source-level operation is an out-of-folio read.  A controlled
UFS1 test with CONFIG_UFS_FS_WRITE enabled also made ufs_find_entry()
return a directory entry from an adjacent anonymous page.  unlink() then
passed that pointer to ufs_delete_entry(), which cleared the adjacent
page's 3
2-bit d_ino.  This write result reproduced three out of three
times.

UFS normally requires a privileged mount path.  A relevant boundary is
a privileged automounter or image-processing service handling an
attacker-supplied filesystem.  The write primitive additionally requires
UFS1 to be mounted read-write with CONFIG_UFS_FS_WRITE enabled; the read
is reachable with read-only UFS2.

Keep the intermediate size arithmetic at 64 bits so non-final pages are
bounded at PAGE_SIZE.  With this change, the same controlled tests reach
the real on-disk directory entry and produce no adjacent-page write or
KASAN report.  A reproducer and full validation logs are available on
request.

The vulnerable helper is present in v7.2-rc3, v7.1, v6.18.38, v6.12.95,
v6.6.111, and upstream master at 1229e2e57a5c.  Runtime reproduction and
fix validation were performed in an x86-64 QEMU guest running v7.2-rc3
with generic KASAN.

Fixes: b71034e5e67d ("[PATCH] ufs: directory and page cache:
 from blocks to pages")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: YANXIN LI <fadouse@xxxxx>
---
 fs/ufs/dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ufs/dir.c b/fs/ufs/dir.c
index e62fe5667..69e6d5535 100644
--- a/fs/ufs/dir.c
+++ b/fs/ufs/dir.c
@@ -213,7 +213,7 @@ static void *ufs_get_folio(struct inode *dir, unsigned long n,
 static unsigned
 ufs_last_byte(struct inode *inode, unsigned long page_nr)
 {
-	unsigned last_byte = inode->i_size;
+	u64 last_byte = inode->i_size;
 
 	last_byte -= page_nr << PAGE_SHIFT;
 	if (last_byte > PAGE_SIZE)

base-commit: 1229e2e57a5c2980ccd457b9b53ea0eed5a22ab3
-- 
2.54.0

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