[PATCH v3 1/3] nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE

From: Robbie Ko <robbieko-UelDjCVBxVpWk0Htik3J/[email protected]>

The NFSv2 sattr decoder converts the wire useconds to nanoseconds in
svcxdr_decode_sattr():

	iap->ia_atime.tv_nsec = tmp2 * NSEC_PER_USEC;

tmp2 is a u32 and NSEC_PER_USEC is 1000, so the product is computed in
unsigned long. On ILP32 that is 32 bits, and an out-of-range useconds
value such as 4294968 wraps to tv_nsec == 704. The corruption therefore
happens during decode, before any proc function can inspect the value,
and a later range check on tv_nsec would see an in-range result and
accept it.

Guard the raw useconds before the multiplication and reject values
greater than 1000000. useconds == 1000000 is kept: it is the Sun
convention for "set to the current server time", and the in-tree Linux
NFSv2 client emits it in both the atime and the mtime field for a plain
touch / utimes(file, NULL) (see encode_sattr() and
xdr_encode_current_server_time() in fs/nfs/nfs2xdr.c). Rejecting 1000000
would turn that common operation into a hard decode failure for both
SETATTR and CREATE. 1000000 * NSEC_PER_USEC is 10^9, which does not wrap
on ILP32, so the Sun convention value passes through safely; only
genuinely out-of-range values (> 1000000) are rejected. The atime and
mtime guards are therefore symmetric.

The decoder only applied the Sun convention in the mtime block, which
clears ATTR_ATIME_SET|ATTR_MTIME_SET when mtime useconds == 1000000. If a
client puts 1000000 in the atime field but not in the mtime field, the
atime block stored an out-of-range tv_nsec (10^9) and left ATTR_ATIME_SET
set, so the bogus value reached the filesystem. Apply the convention in
the atime block as well, clearing ATTR_ATIME_SET so the server uses its
current time and ignores the value. Only ATTR_ATIME_SET is cleared there;
the mtime block keeps its existing behavior, where 1000000 means "set
both atime and mtime to now".

Signed-off-by: Robbie Ko <robbieko-UelDjCVBxVpWk0Htik3J/[email protected]>
---
 fs/nfsd/nfsxdr.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/fs/nfsd/nfsxdr.c b/fs/nfsd/nfsxdr.c
index ae71e0621317..48a4e89a5f41 100644
--- a/fs/nfsd/nfsxdr.c
+++ b/fs/nfsd/nfsxdr.c
@@ -172,14 +172,45 @@ svcxdr_decode_sattr(struct svc_rqst *rqstp, struct xdr_stream *xdr,
 	tmp1 = be32_to_cpup(p++);
 	tmp2 = be32_to_cpup(p++);
 	if (tmp1 != (u32)-1 && tmp2 != (u32)-1) {
+		/*
+		 * Guard the raw useconds before the unit conversion below.
+		 * tmp2 * NSEC_PER_USEC is computed in unsigned long, which is
+		 * 32 bits on ILP32, so an out-of-range value would wrap and
+		 * silently produce a bogus in-range tv_nsec. useconds ==
+		 * 1000000 is the Sun "set to current server time" convention
+		 * (see the mtime block below); allow it and reject anything
+		 * larger. Note 1000000 * NSEC_PER_USEC is 10^9, which does not
+		 * wrap on ILP32.
+		 */
+		if (tmp2 > 1000000)
+			return false;
 		iap->ia_valid |= ATTR_ATIME | ATTR_ATIME_SET;
 		iap->ia_atime.tv_sec = tmp1;
 		iap->ia_atime.tv_nsec = tmp2 * NSEC_PER_USEC;
+		/*
+		 * The Linux NFSv2 client emits useconds == 1000000 in the
+		 * atime field too (touch / utimes(file, NULL) sets ATTR_ATIME
+		 * without ATTR_ATIME_SET). Apply the Sun convention here so
+		 * the server uses its current time and ignores the bogus
+		 * tv_nsec, instead of storing an out-of-range value when the
+		 * mtime field does not also carry 1000000. Only ATTR_ATIME_SET
+		 * is cleared; the mtime block keeps its own handling, where
+		 * 1000000 means "set both atime and mtime to now".
+		 */
+		if (tmp2 == 1000000)
+			iap->ia_valid &= ~ATTR_ATIME_SET;
 	}
 
 	tmp1 = be32_to_cpup(p++);
 	tmp2 = be32_to_cpup(p++);
 	if (tmp1 != (u32)-1 && tmp2 != (u32)-1) {
+		/*
+		 * useconds == 1000000 is a valid Sun convention here (see
+		 * below); anything above that is out of range. Guard it before
+		 * the unit conversion to avoid the ILP32 wrap described above.
+		 */
+		if (tmp2 > 1000000)
+			return false;
 		iap->ia_valid |= ATTR_MTIME | ATTR_MTIME_SET;
 		iap->ia_mtime.tv_sec = tmp1;
 		iap->ia_mtime.tv_nsec = tmp2 * NSEC_PER_USEC;
-- 
2.43.0


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