RE: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation

[David Laight <David.Laight@ACULAB.COM>]

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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