[PATCH v3] 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>
---
V3: Correct inline comments and take Dmitry's suggestion
V2: Correct commit messages and add inline comments

Previous discussion links:
https://lore.kernel.org/lkml/ZlYo20ztfLWPyy5d@xxxxxxxxxx/
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..b7f2fa08d9c8 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 chars, and we are removing
+			 * variable MODALIAS here pointed by @env->envp[i]
+			 * with length @len as shown below:
+			 *
+			 * 0               @env->buf[]      @env->buflen
+			 * ---------------------------------------------
+			 * ^             ^              ^              ^
+			 * |             |->   @len   <-| target block |
+			 * @env->envp[0] @env->envp[i]  @env->envp[i + 1]
+			 *
+			 * so the "target block" indicated above is moved
+			 * backward by @len, and its right size is
+			 * @env->buflen - (@env->envp[i + 1] - @env->envp[0]).
+			 */
 			memmove(env->envp[i], env->envp[i + 1],
-				env->buflen - len);
+				env->buflen - (env->envp[i + 1] - env->envp[0]));
 
 			for (j = i; j < env->envp_idx - 1; j++)
 				env->envp[j] = env->envp[j + 1] - len;
-- 
2.7.4