Messages in this thread | | | From | Dan Williams <> | Date | Fri, 18 May 2018 13:38:58 -0700 | Subject | Re: [PATCH] kernel: sys: fix potential Spectre v1 |
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On Fri, May 18, 2018 at 12:21 PM, Gustavo A. R. Silva <gustavo@embeddedor.com> wrote: > > > On 05/18/2018 02:04 PM, Gustavo A. R. Silva wrote: >> >> >> >> On 05/15/2018 05:57 PM, Dan Williams wrote: >>> >>> On Tue, May 15, 2018 at 3:29 PM, Thomas Gleixner <tglx@linutronix.de> >>> wrote: >>>> >>>> On Tue, 15 May 2018, Andrew Morton wrote: >>>>> >>>>> On Mon, 14 May 2018 22:00:38 -0500 "Gustavo A. R. Silva" >>>>> <gustavo@embeddedor.com> wrote: >>>>> >>>>>> resource can be controlled by user-space, hence leading to a >>>>>> potential exploitation of the Spectre variant 1 vulnerability. >>>>>> >>>>>> This issue was detected with the help of Smatch: >>>>>> >>>>>> kernel/sys.c:1474 __do_compat_sys_old_getrlimit() warn: potential >>>>>> spectre issue 'get_current()->signal->rlim' (local cap) >>>>>> kernel/sys.c:1455 __do_sys_old_getrlimit() warn: potential spectre >>>>>> issue >>>>>> 'get_current()->signal->rlim' (local cap) >>>>>> >>>>>> Fix this by sanitizing *resource* before using it to index >>>>>> current->signal->rlim >>>>>> >>>>>> 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]. >>>>> >>>>> >>>>> hm. Not my area, but I'm always willing to learn ;) >>>>> >>>>>> --- a/kernel/sys.c >>>>>> +++ b/kernel/sys.c >>>>>> @@ -69,6 +69,9 @@ >>>>>> #include <asm/io.h> >>>>>> #include <asm/unistd.h> >>>>>> >>>>>> +/* Hardening for Spectre-v1 */ >>>>>> +#include <linux/nospec.h> >>>>>> + >>>>>> #include "uid16.h" >>>>>> >>>>>> #ifndef SET_UNALIGN_CTL >>>>>> @@ -1451,6 +1454,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned int, >>>>>> resource, >>>>>> if (resource >= RLIM_NLIMITS) >>>>>> return -EINVAL; >>>>>> >>>>>> + resource = array_index_nospec(resource, RLIM_NLIMITS); >>>>>> task_lock(current->group_leader); >>>>>> x = current->signal->rlim[resource]; >>>>> >>>>> >>>>> Can the speculation proceed past the task_lock()? Or is the policy to >>>>> ignore such happy happenstances even if they are available? >>>> >>>> >>>> Locks are not in the way of speculation. Speculation has almost no >>>> limits >>>> except serializing instructions. At least they respect the magic AND >>>> limitation in array_index_nospec(). >>> >>> >>> I'd say it another way, because they don't respect the magic AND, we >>> just make the result in the speculation path safe. So, it's controlled >>> speculation. >>> >> >> Dan, >> >> What do you think about adding the following function to the nospec API: >> >> diff --git a/include/linux/nospec.h b/include/linux/nospec.h >> index e791ebc..81e9a77 100644 >> --- a/include/linux/nospec.h >> +++ b/include/linux/nospec.h >> @@ -55,4 +55,17 @@ static inline unsigned long >> array_index_mask_nospec(unsigned long index, >> \ >> (typeof(_i)) (_i & _mask); \ >> }) >> + >> + >> +#ifndef sanitize_index_nospec >> +inline bool sanitize_index_nospec(unsigned long index, >> + unsigned long size) >> +{ >> + if (index >= size) >> + return false; >> + index = array_index_nospec(index, size); >> + >> + return true; >> +} >> +#endif >> #endif /* _LINUX_NOSPEC_H */ >> > > Oops, it seems I sent the wrong patch. The function would look like this: > > #ifndef sanitize_index_nospec > inline bool sanitize_index_nospec(unsigned long *index, > unsigned long size) > { > if (*index >= size) > return false; > *index = array_index_nospec(*index, size); > > return true; > } > #endif
I think this is fine in concept, we already do something similar in mpls_label_ok(). Perhaps call it validate_index_nospec() since validation is something that can fail, but sanitization in theory is something that can always succeed.
However, the problem is the data type of the index. I expect you would need to do this in a macro and use typeof() if you wanted this to be generally useful, and also watch out for multiple usage of a macro argument. Is it still worth it at that point?
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