[PATCH] security: fix integer overflow in lsm_set_self_attr() syscall

[Paul Moore]

On Feb 14, 2024 Jann Horn <jannh at google.com> wrote:
> 
> security_setselfattr() has an integer overflow bug that leads to
> out-of-bounds access when userspace provides bogus input:
> `lctx->ctx_len + sizeof(*lctx)` is checked against `lctx->len` (and,
> redundantly, also against `size`), but there are no checks on
> `lctx->ctx_len`.
> Therefore, userspace can provide an `lsm_ctx` with `->ctx_len` set to a
> value between `-sizeof(struct lsm_ctx)` and -1, and this bogus `->ctx_len`
> will then be passed to an LSM module as a buffer length, causing LSM
> modules to perform out-of-bounds accesses.
> 
> The following reproducer will demonstrate this under ASAN (if AppArmor is
> loaded as an LSM):
> ```
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <stdint.h>
> #include <stdlib.h>
> #include <sys/syscall.h>
> 
> struct lsm_ctx {
>   uint64_t id;
>   uint64_t flags;
>   uint64_t len;
>   uint64_t ctx_len;
>   char ctx[];
> };
> 
> int main(void) {
>   size_t size = sizeof(struct lsm_ctx);
>   struct lsm_ctx *ctx = malloc(size);
>   ctx->id = 104/*LSM_ID_APPARMOR*/;
>   ctx->flags = 0;
>   ctx->len = size;
>   ctx->ctx_len = -sizeof(struct lsm_ctx);
>   syscall(
>     460/*__NR_lsm_set_self_attr*/,
>     /*attr=*/  100/*LSM_ATTR_CURRENT*/,
>     /*ctx=*/   ctx,
>     /*size=*/  size,
>     /*flags=*/ 0
>   );
> }
> ```
> 
> (I'm including an ASAN splat in the patch notes sent to the list.)
> 
> Fixes: a04a1198088a ("LSM: syscalls for current process attributes")
> Signed-off-by: Jann Horn <jannh at google.com>
> Acked-by: Casey Schaufler <casey at schaufler-ca.com>

Looks good to me, thanks Jann.  I'm going to merge this into
lsm/stable-6.8 and send this up to Linus soon (likely tomorrow).

--
paul-moore.com