Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation | Date | Sat, 29 Aug 2020 13:21:04 +0000 |
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From: Josh Poimboeuf > Sent: 28 August 2020 20:29 > > On Wed, Aug 19, 2020 at 09:50:06AM -0500, Josh Poimboeuf wrote: > > 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). > > > > There are some issues with the current implementation: > > > > - The barrier_nospec() in copy_from_user() was inadvertently removed > > with: 4b842e4e25b1 ("x86: get rid of small constant size cases in > > raw_copy_{to,from}_user()") > > > > - copy_to_user() and friends should also have a speculation barrier, > > because a speculative write to a user-controlled address can still > > populate the cache line with the original data. > > > > - The LFENCE in barrier_nospec() is overkill, when more lightweight user > > pointer masking can be used instead. > > > > Remove all existing barrier_nospec() usage, and instead do user pointer > > masking, throughout the x86 uaccess code. This is similar to what arm64 > > is already doing. > > > > barrier_nospec() is now unused, and can be removed. > > > > Fixes: 4b842e4e25b1 ("x86: get rid of small constant size cases in raw_copy_{to,from}_user()") > > Suggested-by: Will Deacon <will@kernel.org> > > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> > > Ping?
Rereading the patch it looks like a lot of bloat (as well as a lot of changes). Does the array_mask even work on 32bit archs where the kernel base address is 0xc0000000? I'm sure there is something much simpler.
If access_ok() generates ~0u or 0 without a conditional then the address can be masked with the result. So you probably need to change access_ok() to take the address of the user pointer - so the callers become like: if (access_ok(&user_buffer, len)) return -EFAULT __put_user(user_buffer, value);
It would be easier if NULL were guaranteed to be an invalid user address (is it?). Then access_ok() could return the modified pointer. So you get something like: user_buffer = access_ok(user_buffer, len); if (!user_buffer) return -EFAULT.
Provided the 'last' user page is never allocated (it can't be on i386 due to cpu prefetch issues) something like: (and with the asm probably all broken)
static inline void __user * access_ok(void __user *b, size_t len) { unsigned long x = (long)b | (long)(b + len); unsigned long lim = 64_bit ? 1u << 63 : 0x40000000; asm volatile (" add %1, %0\n" " sbb $0, %0", "=r" (x), "r" (lim)); return (void __user *)(long)b & ~x); }
David
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