Re: [PATCH 2/2] x86, tls: Interpret an all-zero struct user_desc as "no segment"

[Borislav Petkov <bp@alien8.de>]

On Thu, Jan 22, 2015 at 11:27:59AM -0800, Andy Lutomirski wrote:
> The Witcher 2 did something like this to allocate a TLS segment index:
> 
>         struct user_desc u_info;
>         bzero(&u_info, sizeof(u_info));
>         u_info.entry_number = (uint32_t)-1;
> 
>         syscall(SYS_set_thread_area, &u_info);
> 
> Strictly speaking, this code was never correct.  It should have set
> read_exec_only and seg_not_present to 1 to indicate that it wanted
> to find a free slot without putting anything there, or it should
> have put something sensible in the TLS slot if it wanted to allocate
> a TLS entry for real.  The actual effect of this code was to
> allocate a bogus segment that could be used to exploit espfix.
> 
> The set_thread_area hardening patches changed the behavior, causing
> set_thread_area to return -EINVAL and crashing the game.
> 
> This changes set_thread_area to interpret this as a request to find
> a free slot and to leave it empty, which isn't *quite* what the game
> expects but should be close enough to keep it working.  In
> particular, using the code above to allocate two segments will
> allocate the same segment both times.
> 
> According to FrostbittenKing on Github, this fixes The Witcher 2.
> 
> If this somehow still causes problems, we could instead allocate
> a limit==0 32-bit data segment, but that seems rather ugly to me.
> 
> Fixes: 41bdc78544b8 x86/tls: Validate TLS entries to protect espfix
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>

Shouldn't this also be CC:stable?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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