Re: [PATCH] tomoyo: common: Fix potential Spectre v1 vulnerability

From: Tetsuo Handa
Date: Thu Aug 01 2019 - 07:03:27 EST


Hello.

Thanks for a patch, but I have a question.

On 2019/08/01 3:54, Gustavo A. R. Silva wrote:
> profile is controlled by user-space via /sys/kernel/security/tomoyo/profile,

It is true that "profile" value is given from user-space, and it will be true
that speculative execution would access "ns->profile_ptr[profile]" before whether
"profile >= TOMOYO_MAX_PROFILES" is true is concluded. But

> hence leading to a potential exploitation of the Spectre variant 1
> vulnerability.

which memory address is vulnerable to Spectre variant 1 attack? How can an attacker
gain information from memory speculatively accessed by "ns->profile_ptr[profile]" ?
Where is the memory access which corresponds to "arr2->data[index2]" demonstrated at
https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html ?

Since I'm not familiar with Spectre/Meltdown problem, this patch sounds as if
"Oh, let's suppress Smatch warning". I want to know whether this problem is real
and this patch is worth keeping stable@xxxxxxxxxxxxxxx ...

> @@ -488,13 +489,15 @@ static void tomoyo_print_number_union(struct tomoyo_io_buffer *head,
> * Returns pointer to "struct tomoyo_profile" on success, NULL otherwise.
> */
> static struct tomoyo_profile *tomoyo_assign_profile
> -(struct tomoyo_policy_namespace *ns, const unsigned int profile)
> +(struct tomoyo_policy_namespace *ns, unsigned int profile)
> {
> struct tomoyo_profile *ptr;
> struct tomoyo_profile *entry;
>
> if (profile >= TOMOYO_MAX_PROFILES)
> return NULL;
> + profile = array_index_nospec(profile, TOMOYO_MAX_PROFILES);
> +
> ptr = ns->profile_ptr[profile];
> if (ptr)
> return ptr;
>

By the way, since /sys/kernel/security/tomoyo/profile is writable by only explicitly
whitelisted domains/programs (&& by only root user by default), I think that it is
OK to treat this "profile" value as trusted.