Messages in this thread Patch in this message | | | Date | Wed, 5 May 2021 08:19:43 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation |
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On Wed, May 05, 2021 at 08:48:48AM +0000, David Laight wrote: > From: Josh Poimboeuf > > Sent: 05 May 2021 04:55 > > > > The x86 uaccess code uses barrier_nospec() in various places to prevent > > speculative dereferencing of user-controlled pointers (which might be > > combined with further gadgets or CPU bugs to leak data). > ... > > Remove existing barrier_nospec() usage, and instead do user pointer > > masking, throughout the x86 uaccess code. This is similar to what arm64 > > is already doing with uaccess_mask_ptr(). > ... > > diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h > > index fb75657b5e56..ebe9ab46b183 100644 > > --- a/arch/x86/include/asm/uaccess.h > > +++ b/arch/x86/include/asm/uaccess.h > > @@ -66,12 +66,35 @@ static inline bool pagefault_disabled(void); > > * Return: true (nonzero) if the memory block may be valid, false (zero) > > * if it is definitely invalid. > > */ > > -#define access_ok(addr, size) \ > > +#define access_ok(addr, size) \ > > ({ \ > > WARN_ON_IN_IRQ(); \ > > likely(!__range_not_ok(addr, size, TASK_SIZE_MAX)); \ > > }) > > > > +/* > > + * Sanitize a user pointer such that it becomes NULL if it's not a valid user > > + * pointer. This prevents speculatively dereferencing a user-controlled > > + * pointer to kernel space if access_ok() speculatively returns true. This > > + * should be done *after* access_ok(), to avoid affecting error handling > > + * behavior. > > + */ > > +#define mask_user_ptr(ptr) \ > > +({ \ > > + unsigned long _ptr = (__force unsigned long)ptr; \ > > + unsigned long mask; \ > > + \ > > + asm volatile("cmp %[max], %[_ptr]\n\t" \ > > + "sbb %[mask], %[mask]\n\t" \ > > + : [mask] "=r" (mask) \ > > + : [_ptr] "r" (_ptr), \ > > + [max] "r" (TASK_SIZE_MAX) \ > > + : "cc"); \ > > + \ > > + mask &= _ptr; \ > > + ((typeof(ptr)) mask); \ > > +}) > > + > > access_ok() and mask_user_ptr() are doing much the same check. > Is there scope for making access_ok() return the masked pointer? > > So the canonical calling code would be: > uptr = access_ok(uptr, size); > if (!uptr) > return -EFAULT; > > This would error requests for address 0 earlier - but I don't > believe they are ever valid in Linux. > (Some historic x86 a.out formats did load to address 0.) > > Clearly for a follow up patch.
Yeah. I mentioned a similar idea in the cover letter.
But I'm thinking we should still rename it to access_ok_mask(), or otherwise change the API to avoid the masked value getting ignored.
But that'll be a much bigger patch.
-- Josh
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