From: David Laight <David.Laight@ACULAB.COM>
To: 'Andy Lutomirski' <luto@amacapital.net>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
Andy Lutomirsky <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Dave Hansen <dave.hansen@intel.com>,
Borislav Petkov <bpetkov@suse.de>,
Greg KH <gregkh@linuxfoundation.org>,
Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>,
Brian Gerst <brgerst@gmail.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Denys Vlasenko <dvlasenk@redhat.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Juergen Gross <jgross@suse.com>,
Eduardo Valentin <eduval@amazon.com>,
"Liguori, Anthony" <aliguori@amazon.com>,
Will Deacon <will.deacon@arm.com>, linux-mm <linux-mm@kvack.org>
Subject: RE: [patch 11/16] x86/ldt: Force access bit for CS/SS
Date: Tue, 19 Dec 2017 12:10:34 +0000 [thread overview]
Message-ID: <2956b9271ecf4a859ef8eb112940cd3b@AcuMS.aculab.com> (raw)
In-Reply-To: <BF0E88FD-9438-4ABF-82BD-AA634F957C3D@amacapital.net>
From: Andy Lutomirski
> Sent: 12 December 2017 19:27
...
> > Why is the iret exception unrecoverable anyway? Does anybody even know?
> >
>
> Weird microcode shit aside, a fault on IRET will return to kernel code with kernel GS, and then the
> next time we enter the kernel we're backwards. We could fix idtentry to get this right, but the code
> is already tangled enough.
...
Notwithstanding a readonly LDT, the iret (and pop %ds, pop %es that probably
precede it) are all likely to fault in kernel if the segment registers are invalid.
(Setting %fs and %gs for 32 bit processes is left to the reader.)
Unlike every other fault in the kernel code segment, gsbase will contain
the user value, not the kernel one.
The kernel code must detect this somehow and correct everything before (probably)
generating a SIGSEGV and returning to the user's signal handler with the
invalid segment registers in the signal context.
Assuming this won't happen (because the segment registers are always valid)
is likely to be a recipe for disaster (or an escalation).
I guess the problem with a readonly LDT is that you don't want to fault
setting the 'accesses' bit.
David
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next prev parent reply other threads:[~2017-12-19 12:10 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-12 17:32 [patch 00/16] x86/ldt: Use a VMA based read only mapping Thomas Gleixner
2017-12-12 17:32 ` [patch 01/16] arch: Allow arch_dup_mmap() to fail Thomas Gleixner
2017-12-12 17:32 ` [patch 02/16] x86/ldt: Rework locking Thomas Gleixner
2017-12-12 17:32 ` [patch 03/16] x86/ldt: Prevent ldt inheritance on exec Thomas Gleixner
2017-12-12 17:32 ` [patch 04/16] mm/softdirty: Move VM_SOFTDIRTY into high bits Thomas Gleixner
2017-12-12 17:32 ` [patch 05/16] mm: Allow special mappings with user access cleared Thomas Gleixner
2017-12-12 18:00 ` Andy Lutomirski
2017-12-12 18:05 ` Peter Zijlstra
2017-12-12 18:06 ` Andy Lutomirski
2017-12-12 18:25 ` Peter Zijlstra
2017-12-13 12:22 ` Peter Zijlstra
2017-12-13 12:57 ` Kirill A. Shutemov
2017-12-13 14:34 ` Peter Zijlstra
2017-12-13 14:43 ` Kirill A. Shutemov
2017-12-13 15:00 ` Peter Zijlstra
2017-12-13 15:04 ` Peter Zijlstra
2017-12-13 15:14 ` Dave Hansen
2017-12-13 15:32 ` Peter Zijlstra
2017-12-13 15:47 ` Dave Hansen
2017-12-13 15:54 ` Peter Zijlstra
2017-12-13 18:08 ` Linus Torvalds
2017-12-13 18:21 ` Dave Hansen
2017-12-13 18:23 ` Linus Torvalds
2017-12-13 18:31 ` Andy Lutomirski
2017-12-13 18:32 ` Peter Zijlstra
2017-12-13 18:35 ` Linus Torvalds
2017-12-14 4:53 ` Aneesh Kumar K.V
2017-12-13 21:50 ` Matthew Wilcox
2017-12-13 22:12 ` Peter Zijlstra
2017-12-14 0:10 ` Matthew Wilcox
2017-12-14 0:16 ` Andy Lutomirski
2017-12-12 17:32 ` [patch 06/16] mm: Provide vm_special_mapping::close Thomas Gleixner
2017-12-12 17:32 ` [patch 07/16] selftest/x86: Implement additional LDT selftests Thomas Gleixner
2017-12-12 17:32 ` [patch 08/16] selftests/x86/ldt_gdt: Prepare for access bit forced Thomas Gleixner
2017-12-12 17:32 ` [patch 09/16] mm: Make populate_vma_page_range() available Thomas Gleixner
2017-12-12 17:32 ` [patch 10/16] x86/ldt: Do not install LDT for kernel threads Thomas Gleixner
2017-12-12 17:57 ` Andy Lutomirski
2017-12-12 17:32 ` [patch 11/16] x86/ldt: Force access bit for CS/SS Thomas Gleixner
2017-12-12 18:03 ` Andy Lutomirski
2017-12-12 18:09 ` Peter Zijlstra
2017-12-12 18:10 ` Andy Lutomirski
2017-12-12 18:22 ` Andy Lutomirski
2017-12-12 18:29 ` Peter Zijlstra
2017-12-12 18:41 ` Thomas Gleixner
2017-12-12 19:04 ` Peter Zijlstra
2017-12-12 19:05 ` Linus Torvalds
2017-12-12 19:26 ` Andy Lutomirski
2017-12-19 12:10 ` David Laight [this message]
2017-12-12 17:32 ` [patch 12/16] x86/ldt: Reshuffle code Thomas Gleixner
2017-12-12 17:32 ` [patch 13/16] x86/ldt: Introduce LDT write fault handler Thomas Gleixner
2017-12-12 17:58 ` Andy Lutomirski
2017-12-12 18:19 ` Peter Zijlstra
2017-12-12 18:43 ` Thomas Gleixner
2017-12-12 19:01 ` Linus Torvalds
2017-12-12 19:21 ` Thomas Gleixner
2017-12-12 19:51 ` Linus Torvalds
2017-12-12 20:21 ` Dave Hansen
2017-12-12 20:37 ` Thomas Gleixner
2017-12-12 21:35 ` Andy Lutomirski
2017-12-12 21:42 ` Thomas Gleixner
2017-12-12 21:41 ` Thomas Gleixner
2017-12-12 21:46 ` Thomas Gleixner
2017-12-12 22:25 ` Peter Zijlstra
2017-12-12 17:32 ` [patch 14/16] x86/ldt: Prepare for VMA mapping Thomas Gleixner
2017-12-12 17:32 ` [patch 15/16] x86/ldt: Add VMA management code Thomas Gleixner
2017-12-12 17:32 ` [patch 16/16] x86/ldt: Make it read only VMA mapped Thomas Gleixner
2017-12-12 18:03 ` [patch 00/16] x86/ldt: Use a VMA based read only mapping Andy Lutomirski
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