[PATCH v7 2/5] s390/sclp: Introduce macro sclp_gds_for_each()

[Alexander Egorenkov <egorenar@linux.ibm.com>]

sclp_find_gds_{sub}vector() does not deal well with malformed event buffers
consisting of GDS {sub}vectors. This can result in an infinite loop or
an out-of-bounds memory read. Therefore, abort with NULL if
* the next GDS header would exceed the given end boundary
* the length in a GDS header contains an invalid value.
  A valid length value in a GDS header should be at least as large
  as the size of the corresponding GDS header (2 or 4 bytes)
  but also not lead to exceeding the given end boundary.

Use the new macro in sclp_find_gds_{sub}vector() to iterate over entries
of a GDS {sub}vector in a safe manner bailing out on the first invalid
entry.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Fixes: 30c2df51173e ("[S390] sclp: event buffer dissection")
---
 drivers/s390/char/sclp.h | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/s390/char/sclp.h b/drivers/s390/char/sclp.h
index b31a680e0871..a006d7abfdf3 100644
--- a/drivers/s390/char/sclp.h
+++ b/drivers/s390/char/sclp.h
@@ -360,25 +360,33 @@ sclp_ascebc_str(char *str, int nr)
 	(machine_is_vm()) ? ASCEBC(str, nr) : ASCEBC_500(str, nr);
 }
 
-static inline struct gds_vector *
-sclp_find_gds_vector(void *start, void *end, u16 id)
+/* Loop over all GDS {sub}vectors in a safe manner. */
+#define sclp_gds_for_each(v, n, start, end)	\
+	for ((n) = (void*)(end) - (void*)(start), (v) = (typeof(v))(start);	\
+	     (n) >= sizeof(*(v)) && (v)->length >= sizeof(*(v)) && (v)->length <= (n);	\
+	     (n) -= (v)->length, (v) = (void*)(v) + (v)->length)
+
+static inline struct gds_vector *sclp_find_gds_vector(void *start,
+						      void *end, u16 id)
 {
 	struct gds_vector *v;
-
-	for (v = start; (void *) v < end; v = (void *) v + v->length)
+	int n;
+	sclp_gds_for_each(v, n, start, end) {
 		if (v->gds_id == id)
 			return v;
+	}
 	return NULL;
 }
 
-static inline struct gds_subvector *
-sclp_find_gds_subvector(void *start, void *end, u8 key)
+static inline struct gds_subvector *sclp_find_gds_subvector(void *start,
+							    void *end, u8 key)
 {
 	struct gds_subvector *sv;
-
-	for (sv = start; (void *) sv < end; sv = (void *) sv + sv->length)
+	int n;
+	sclp_gds_for_each(sv, n, start, end) {
 		if (sv->key == key)
 			return sv;
+	}
 	return NULL;
 }
 
-- 
2.53.0