Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] x86/uaccess: Use pointer masking to limit uaccess speculation | Date | Tue, 1 Sep 2020 08:32:20 +0000 |
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From: Josh Poimboeuf > Sent: 31 August 2020 18:31 > On Sat, Aug 29, 2020 at 07:31:20PM +0000, David Laight wrote: > > > 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? > > Why wouldn't it on work on 32-bit? My patch does have a minor compile > bug on 32-bit, but otherwise it seems to work (i.e., the asm looks ok, > and it boots).
As usual I hadn't looked closely enough into the masked_array internals.
... > > Actually, thinking further, if: > > 1) the access_ok() immediately precedes the user copy (as it should). > > 2) the user-copies use a sensible 'increasing address' copy. > > and > > 3) there is a 'guard page' between valid user and kernel addresses. > > Then access_ok() only need check the base address of the user buffer. > > Yes, it would make sense to put the masking in access_ok() somehow. But > to do it properly, I think we'd first need to make access_ok() generic. > Maybe that's do-able, but it would be a much bigger patch set. > > First I'd prefer to just fix x86, like my patch does. Then we could do > an access_ok() rework.
If you do a modified access_ok() you get to (slowly) collect all the important paths. No point replicating the same test.
A lot of the access_ok() can be deleted - maybe remove some __ from the following functions. Or change to the variants that enable user-space accesses.
David
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