[PATCH 4/4] xfs: pin inodes that would otherwise overflow link count

Darrick J. Wong
2023-12-31T22:36:26Z

The VFS inc_nlink function does not explicitly check for integer
overflows in the i_nlink field. Instead, it checks the link count
against s_max_links in the vfs_{link,create,rename} functions. XFS
sets the maximum link count to 2.1 billion, so integer overflows should
not be a problem.

However. It's possible that online repair could find that a file has
more than four billion links, particularly if the link count got
corrupted while creating hardlinks to the file. The di_nlinkv2 field is
not large enough to store a value larger than 2^32, so we ought to
define a magic pin value of ~0U which means that the inode never gets
deleted. This will prevent a UAF error if the repair finds this
situation and users begin deleting links to the file.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
libxfs/util.c | 3 ++-
libxfs/xfs_format.h | 6 ++++++
repair/incore_ino.c | 3 ++-
3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/libxfs/util.c b/libxfs/util.c
index 11978529ed6..03191ebcd08 100644
--- a/libxfs/util.c
+++ b/libxfs/util.c
@@ -252,7 +252,8 @@ libxfs_bumplink(

xfs_trans_ichgtime(tp, ip, XFS_ICHGTIME_CHG);

- inc_nlink(inode);
+ if (inode->i_nlink != XFS_NLINK_PINNED)
+ inc_nlink(inode);

xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
}
diff --git a/libxfs/xfs_format.h b/libxfs/xfs_format.h
index 7861539ab8b..ec25010b577 100644
--- a/libxfs/xfs_format.h
+++ b/libxfs/xfs_format.h
@@ -912,6 +912,12 @@ static inline uint xfs_dinode_size(int version)
*/
#define XFS_MAXLINK ((1U << 31) - 1U)

+/*
+ * Any file that hits the maximum ondisk link count should be pinned to avoid
+ * a use-after-free situation.
+ */
+#define XFS_NLINK_PINNED (~0U)
+
/*
* Values for di_format
*
diff --git a/repair/incore_ino.c b/repair/incore_ino.c
index 0dd7a2f060f..b0b41a2cc5c 100644
--- a/repair/incore_ino.c
+++ b/repair/incore_ino.c
@@ -108,7 +108,8 @@ void add_inode_ref(struct ino_tree_node *irec, int ino_offset)
nlink_grow_16_to_32(irec);
/*FALLTHRU*/
case sizeof(uint32_t):
- irec->ino_un.ex_data->counted_nlinks.un32[ino_offset]++;
+ if (irec->ino_un.ex_data->counted_nlinks.un32[ino_offset] != XFS_NLINK_PINNED)
+ irec->ino_un.ex_data->counted_nlinks.un32[ino_offset]++;
break;
default:
ASSERT(0);