NFS: Fix shift-out-of-bounds UBSAN warning with negative retrans

Commit Message

Wang Zhaolong April 7, 2025, 1:48 p.m. UTC

The previous commit c09f11ef3595 ("NFS: fs_context: validate UDP retrans
to prevent shift out-of-bounds") added a check to prevent shift values
that are too large, but it fails to account for negative retrans values.
When data->retrans is negative, the condition `data->retrans >= 64` is
skipped, allowing negative values to be copied to context->retrans,
which is unsigned. This results in a large positive number that can
trigger the original UBSAN issue[1].

This patch modifies the check to explicitly handle both negative values
and values that are too large.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=219988
Fixes: 9954bf92c0cd ("NFS: Move mount parameterisation bits into their own file")
Signed-off-by: Wang Zhaolong <[e-mail]>
---
 fs/nfs/fs_context.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Patch

diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
index 13f71ca8c974..0703ac0349cb 100644
--- a/fs/nfs/fs_context.c
+++ b/fs/nfs/fs_context.c
@@ -1161,11 +1161,12 @@  static int nfs23_parse_monolithic(struct fs_context *fc,
 		 * for proto == XPRT_TRANSPORT_UDP, which is what uses
 		 * to_exponential, implying shift: limit the shift value
 		 * to BITS_PER_LONG (majortimeo is unsigned long)
 		 */
 		if (!(data->flags & NFS_MOUNT_TCP)) /* this will be UDP */
-			if (data->retrans >= 64) /* shift value is too large */
+			/* Reject invalid retrans values (negative or too large) */
+			if (data->retrans < 0 || data->retrans >= 64)
 				goto out_invalid_data;
 
 		/*
 		 * Translate to nfs_fs_context, which nfs_fill_super
 		 * can deal with.