Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 5 May 2021 09:55:11 -0700 | Subject | Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation |
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On Tue, May 4, 2021 at 8:55 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote: > +/* > + * 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); \ > +})
Is there an equally efficient sequence that squishes the pointer value to something noncanonical or something like -1 instead of 0? I'm not sure this matters, but it opens up the possibility of combining the access_ok check with the masking without any branches at all.
Also, why are you doing mask &= _ptr; mask instead of just ((typeof(ptr)) (_ptr & mask))? or _ptr &= mask, for that matter?
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