diff options
Diffstat (limited to 'arch/x86/include/asm')
| -rw-r--r-- | arch/x86/include/asm/uaccess_64.h | 44 |
1 files changed, 31 insertions, 13 deletions
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h index e5b23e917f41..81b826d3b753 100644 --- a/arch/x86/include/asm/uaccess_64.h +++ b/arch/x86/include/asm/uaccess_64.h @@ -50,27 +50,45 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, #endif /* - * On x86-64, we may have tag bits in the user pointer. Rather than - * mask them off, just change the rules for __access_ok(). + * The virtual address space space is logically divided into a kernel + * half and a user half. When cast to a signed type, user pointers + * are positive and kernel pointers are negative. + */ +#define valid_user_address(x) ((long)(x) >= 0) + +/* + * User pointers can have tag bits on x86-64. This scheme tolerates + * arbitrary values in those bits rather then masking them off. + * + * Enforce two rules: + * 1. 'ptr' must be in the user half of the address space + * 2. 'ptr+size' must not overflow into kernel addresses + * + * Note that addresses around the sign change are not valid addresses, + * and will GP-fault even with LAM enabled if the sign bit is set (see + * "CR3.LAM_SUP" that can narrow the canonicality check if we ever + * enable it, but not remove it entirely). + * + * So the "overflow into kernel addresses" does not imply some sudden + * exact boundary at the sign bit, and we can allow a lot of slop on the + * size check. * - * Make the rule be that 'ptr+size' must not overflow, and must not - * have the high bit set. Compilers generally understand about - * unsigned overflow and the CF bit and generate reasonable code for - * this. Although it looks like the combination confuses at least - * clang (and instead of just doing an "add" followed by a test of - * SF and CF, you'll see that unnecessary comparison). + * In fact, we could probably remove the size check entirely, since + * any kernel accesses will be in increasing address order starting + * at 'ptr', and even if the end might be in kernel space, we'll + * hit the GP faults for non-canonical accesses before we ever get + * there. * - * For the common case of small sizes that can be checked at compile - * time, don't even bother with the addition, and just check that the - * base pointer is ok. + * That's a separate optimization, for now just handle the small + * constant case. */ static inline bool __access_ok(const void __user *ptr, unsigned long size) { if (__builtin_constant_p(size <= PAGE_SIZE) && size <= PAGE_SIZE) { - return (long)ptr >= 0; + return valid_user_address(ptr); } else { unsigned long sum = size + (unsigned long)ptr; - return (long) sum >= 0 && sum >= (unsigned long)ptr; + return valid_user_address(sum) && sum >= (unsigned long)ptr; } } #define __access_ok __access_ok |