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

Paul Moore
2024-02-14T18:53:15Z

On Feb 14, 2024 Jann Horn <jannh@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@google.com>
> Acked-by: Casey Schaufler <casey@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