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Date: Thu, 28 Jun 2018 14:59:25 +0200
From: Solar Designer <solar@...nwall.com>
To: owl-dev@...ts.openwall.com
Cc: Vasily Averin <vvs@...tuozzo.com>
Subject: Re: 32-bit syscall breakage in -431 kernel with KAISER
On Tue, Jun 26, 2018 at 09:13:43PM +0200, Solar Designer wrote:
> per my review of the full struct tss_struct, the stack[] field
> offset is:
>
> 4+8*5+4*2+2*2+1025*8+8 = 8264
>
> So it's not 16-byte aligned. But that's the start of stack[], and we
> need the stack pointer to be aligned. Yet I suppose aligning stack[]
> itself is in fact the most appropriate fix.
>
> Reviewing the diffs between e.g. -423 and -431, I see this stack[] in
> struct tss is in fact newly added:
At first glance, RHEL6 might also be affected. From 2.6.32-754.el6 as
currently used by that OpenVZ branch:
struct x86_hw_tss {
u32 reserved1;
u64 sp0;
u64 sp1;
u64 sp2;
u64 reserved2;
u64 ist[7];
u32 reserved3;
u32 reserved4;
u16 reserved5;
u16 io_bitmap_base;
} __attribute__((packed)) ____cacheline_aligned;
This is 4+8*5+4*2+2*2 = 56 bytes like above.
struct tss_struct {
/*
* The hardware state:
*/
struct x86_hw_tss x86_tss;
/*
* The extra 1 is there because the CPU will access an
* additional byte beyond the end of the IO permission
* bitmap. The extra byte must be all 1 bits, and must
* be within the limit.
*/
unsigned long io_bitmap[IO_BITMAP_LONGS + 1];
/*
* .. and then another 0x100 bytes for the emergency kernel stack:
*/
#ifndef __GENKSYMS__
unsigned long stack_canary;
unsigned long stack[256];
#else
unsigned long stack[64];
#endif
/*
*
* The Intel SDM says (Volume 3, 7.2.1):
*
* Avoid placing a page boundary in the part of the TSS that the
* processor reads during a task switch (the first 104 bytes). The
* processor may not correctly perform address translations if a
* boundary occurs in this area. During a task switch, the processor
* reads and writes into the first 104 bytes of each TSS (using
* contiguous physical addresses beginning with the physical address
* of the first byte of the TSS). So, after TSS access begins, if
* part of the 104 bytes is not physically contiguous, the processor
* will access incorrect information without generating a page-fault
* exception.
*
* There are also a lot of errata involving the TSS spanning a page
* boundary. Assert that we're not doing that.
*/
} __attribute__((__aligned__(PAGE_SIZE)));
This second struct lacks __attribute__((packed)), but since 56 is a
multiple of 8 and since everything after it appears to only require an
8-byte alignment as far as gcc is concerned, I'd expect stack[] to be
misaligned just like we think it is on RHEL5 - except if this is built
with __GENKSYMS__. Is it?
In that kernel, the stack_canary check is inside "#ifdef
CONFIG_DEBUG_STACKOVERFLOW", so if that option isn't enabled then I
guess this kernel would build with __GENKSYMS__.
Alexander
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