[PATCH v2] kobject_uevent: Fix OOB access within zap_modalias_env()

[Zijun Hu]

zap_modalias_env() wrongly calculates size of memory block to move, so
will cause OOB memory access issue if variable MODALIAS is not the last
one within its @env parameter, fixed by correcting size to memmove.

Fixes: 9b3fa47d4a76 ("kobject: fix suppressing modalias in uevents delivered over netlink")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
---
V1 -> V2: Correct commit messages and add inline comments

V1 discussion link:
https://lore.kernel.org/lkml/0b916393-eb39-4467-9c99-ac1bc9746512@xxxxxxxxxxx/T/#m8d80165294640dbac72f5c48d14b7ca4f097b5c7

 lib/kobject_uevent.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index 03b427e2707e..f22366be020c 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -433,8 +433,23 @@ static void zap_modalias_env(struct kobj_uevent_env *env)
 		len = strlen(env->envp[i]) + 1;
 
 		if (i != env->envp_idx - 1) {
+			/* @env->envp[] contains pointers to @env->buf[]
+			 * with @env->buflen elements, and we want to
+			 * remove variable MODALIAS pointed by
+			 * @env->envp[i] with length @len as shown below:
+			 *
+			 * 0          @env->buf[]      @env->buflen
+			 * ----------------------------------------
+			 *      ^              ^                  ^
+			 *      |->   @len   <-|   target block   |
+			 * @env->envp[i]  @env->envp[i+1]
+			 *
+			 * so the "target block" indicated above is moved
+			 * backward by @len, and its right size is
+			 * (@env->buf + @env->buflen - @env->envp[i + 1]).
+			 */
 			memmove(env->envp[i], env->envp[i + 1],
-				env->buflen - len);
+				env->buf + env->buflen - env->envp[i + 1]);
 
 			for (j = i; j < env->envp_idx - 1; j++)
 				env->envp[j] = env->envp[j + 1] - len;
-- 
2.7.4