[PATCH net] slip: bound decode() reads against the compressed packet length

[Weiming Shi <bestswngs@gmail.com>]

slhc_uncompress() parses a VJ-compressed TCP header by advancing a
pointer through the packet via decode() and pull16(). Neither helper
bounds-checks against isize, and decode() masks its return with
& 0xffff so it can never return the -1 that callers test for -- those
error paths are dead code.

A short compressed frame whose change byte requests optional fields
lets decode() read past the end of the packet. The over-read bytes
are folded into the cached cstate and reflected into subsequent
reconstructed packets.

Make decode() and pull16() take the packet end pointer and return -1
when exhausted. Add a bounds check before the TCP-checksum read.
The existing == -1 tests now do what they were always meant to.

Fixes: b5451d783ade ("slip: Move the SLIP drivers")
Reported-by: Simon Horman <horms@kernel.org>
Closes: https://lore.kernel.org/netdev/20260414134126.758795-2-horms@kernel.org/
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 drivers/net/slip/slhc.c | 43 ++++++++++++++++++++++++-----------------
 1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/drivers/net/slip/slhc.c b/drivers/net/slip/slhc.c
index e3c785da3eef3..d1c0523085d3a 100644
--- a/drivers/net/slip/slhc.c
+++ b/drivers/net/slip/slhc.c
@@ -80,9 +80,9 @@
 #include <linux/unaligned.h>
 
 static unsigned char *encode(unsigned char *cp, unsigned short n);
-static long decode(unsigned char **cpp);
+static long decode(unsigned char **cpp, const unsigned char *end);
 static unsigned char * put16(unsigned char *cp, unsigned short x);
-static unsigned short pull16(unsigned char **cpp);
+static long pull16(unsigned char **cpp, const unsigned char *end);
 
 /* Allocate compression data structure
  *	slots must be in range 0 to 255 (zero meaning no compression)
@@ -190,30 +190,34 @@ encode(unsigned char *cp, unsigned short n)
 	return cp;
 }
 
-/* Pull a 16-bit integer in host order from buffer in network byte order */
-static unsigned short
-pull16(unsigned char **cpp)
+/* Pull a 16-bit integer in host order from buffer in network byte order.
+ * Returns -1 if the buffer is exhausted, otherwise the 16-bit value.
+ */
+static long
+pull16(unsigned char **cpp, const unsigned char *end)
 {
-	short rval;
+	long rval;
 
+	if (*cpp + 2 > end)
+		return -1;
 	rval = *(*cpp)++;
 	rval <<= 8;
 	rval |= *(*cpp)++;
 	return rval;
 }
 
-/* Decode a number */
+/* Decode a number. Returns -1 if the buffer is exhausted. */
 static long
-decode(unsigned char **cpp)
+decode(unsigned char **cpp, const unsigned char *end)
 {
 	int x;
 
+	if (*cpp >= end)
+		return -1;
 	x = *(*cpp)++;
-	if(x == 0){
-		return pull16(cpp) & 0xffff;	/* pull16 returns -1 on error */
-	} else {
-		return x & 0xff;		/* -1 if PULLCHAR returned error */
-	}
+	if (x == 0)
+		return pull16(cpp, end);
+	return x & 0xff;
 }
 
 /*
@@ -499,6 +503,7 @@ slhc_uncompress(struct slcompress *comp, unsigned char *icp, int isize)
 	struct cstate *cs;
 	int len, hdrlen;
 	unsigned char *cp = icp;
+	const unsigned char *end = icp + isize;
 
 	/* We've got a compressed packet; read the change byte */
 	comp->sls_i_compressed++;
@@ -534,6 +539,8 @@ slhc_uncompress(struct slcompress *comp, unsigned char *icp, int isize)
 	thp = &cs->cs_tcp;
 	ip = &cs->cs_ip;
 
+	if (cp + 2 > end)
+		goto bad;
 	thp->check = *(__sum16 *)cp;
 	cp += 2;
 
@@ -564,26 +571,26 @@ slhc_uncompress(struct slcompress *comp, unsigned char *icp, int isize)
 	default:
 		if(changes & NEW_U){
 			thp->urg = 1;
-			if((x = decode(&cp)) == -1) {
+			if((x = decode(&cp, end)) == -1) {
 				goto bad;
 			}
 			thp->urg_ptr = htons(x);
 		} else
 			thp->urg = 0;
 		if(changes & NEW_W){
-			if((x = decode(&cp)) == -1) {
+			if((x = decode(&cp, end)) == -1) {
 				goto bad;
 			}
 			thp->window = htons( ntohs(thp->window) + x);
 		}
 		if(changes & NEW_A){
-			if((x = decode(&cp)) == -1) {
+			if((x = decode(&cp, end)) == -1) {
 				goto bad;
 			}
 			thp->ack_seq = htonl( ntohl(thp->ack_seq) + x);
 		}
 		if(changes & NEW_S){
-			if((x = decode(&cp)) == -1) {
+			if((x = decode(&cp, end)) == -1) {
 				goto bad;
 			}
 			thp->seq = htonl( ntohl(thp->seq) + x);
@@ -591,7 +598,7 @@ slhc_uncompress(struct slcompress *comp, unsigned char *icp, int isize)
 		break;
 	}
 	if(changes & NEW_I){
-		if((x = decode(&cp)) == -1) {
+		if((x = decode(&cp, end)) == -1) {
 			goto bad;
 		}
 		ip->id = htons (ntohs (ip->id) + x);
-- 
2.43.0