Re: [PATCH] tomoyo: common: Fix potential Spectre v1 vulnerability
From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Date: 2019-08-01 11:03:27
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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@vger.kernel.org ...
quoted hunk ↗ jump to hunk
@@ -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.