tomoyo: common: Fix potential Spectre v1 vulnerability
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Message ID 20190731185457.GA21407@embeddedor
State New
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Series
  • tomoyo: common: Fix potential Spectre v1 vulnerability
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Commit Message

Gustavo A. R. Silva July 31, 2019, 6:54 p.m. UTC
profile is controlled by user-space via /sys/kernel/security/tomoyo/profile,
hence leading to a potential exploitation of the Spectre variant 1
vulnerability.

This issue was detected with the help of Smatch:

security/tomoyo/common.c:498 tomoyo_assign_profile() warn: potential spectre issue 'ns->profile_ptr' [r] (local cap)
security/tomoyo/common.c:499 tomoyo_assign_profile() warn: possible spectre second half.  'ptr'
security/tomoyo/common.c:505 tomoyo_assign_profile() warn: possible spectre second half.  'ptr'
security/tomoyo/common.c:523 tomoyo_assign_profile() warn: possible spectre second half.  'ptr'

Fix this by sanitizing profile before using it to index ns->profile_ptr

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://lore.kernel.org/lkml/20180423164740.GY17484@dhcp22.suse.cz/

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 security/tomoyo/common.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Comments

Tetsuo Handa Aug. 1, 2019, 11:02 a.m. UTC | #1
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 ...

> @@ -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.

Patch
diff mbox series

diff --git a/security/tomoyo/common.c b/security/tomoyo/common.c
index dd3d5942e669..45858dbcfdb9 100644
--- a/security/tomoyo/common.c
+++ b/security/tomoyo/common.c
@@ -8,6 +8,7 @@ 
 #include <linux/uaccess.h>
 #include <linux/slab.h>
 #include <linux/security.h>
+#include <linux/nospec.h>
 #include "common.h"
 
 /* String table for operation mode. */
@@ -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;