Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation | Date | Thu, 6 May 2021 07:57:44 +0000 |
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From: Linus Torvalds > Sent: 05 May 2021 19:32 > > On Wed, May 5, 2021 at 1:48 AM David Laight <David.Laight@aculab.com> wrote: > > > > This would error requests for address 0 earlier - but I don't > > believe they are ever valid in Linux. > > (Some historic x86 a.out formats did load to address 0.) > > Not only loading at address 0 - there are various real reason s why > address 0 might actually be needed. > > Anybody who still runs a 32-bit kernel and wants to use vm86 mode, for > example, requires address 0 because that's simply how the hardware > works. > > So no. "mask to zero and make zero invalid" is not a proper model.
I had my doubts. But letting userspace map address zero has been a security problem. It can turn a kernel panic into executing 'user' code with supervisor permissions.
So I did wonder if it had been banned completely.
David
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