linux-kernel - Re: [PATCH] x86/coredump: always use user_regs_struct for compat_elf_gregset

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Message-ID: <20161209035246.GB30637@gmail.com>
Date:   Fri, 9 Dec 2016 04:52:46 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dmitry Safonov <dsafonov@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/coredump: always use user_regs_struct for
 compat_elf_gregset_t


* Andy Lutomirski <luto@...capital.net> wrote:

> On Nov 23, 2016 10:16 AM, "Dmitry Safonov" <dsafonov@...tuozzo.com> wrote:
> >
> > From commit 90954e7b9407 ("x86/coredump: Use pr_reg size, rather that
> > TIF_IA32 flag") elf coredump file is constructed according to register
> > set size - and that's good: if binary crashes with 32-bit code selector,
> > generate 32-bit ELF core, otherwise - 64-bit core.
> > That was made for restoring 32-bit applications on x86_64: we want
> > 32-bit application after restore to generate 32-bit ELF dump on crash.
> > All was quite good and recently I started reworking 32-bit applications
> > dumping part of CRIU: now it has two parasites (32 and 64) for seizing
> > compat/native tasks, after rework it'll have one parasite, working in
> > 64-bit mode, to which 32-bit prologue long-jumps during infection.
> >
> > And while it has worked for my work machine, in VM with
> > !CONFIG_X86_X32_ABI during reworking I faced that segfault in 32-bit
> > binary, that has long-jumped to 64-bit mode results in dereference
> > of garbage:
> 
> Can you point to the actual line that's crashing?  I'm wondering if we
> have code that should be made more robust.

Agreed. Note that because it fixes a crash this fix is now upstream:

 Commit-ID:  7b2dd3682896bcf1abbbbe870885728db2832a3c
 Gitweb:     http://git.kernel.org/tip/7b2dd3682896bcf1abbbbe870885728db2832a3c
 Author:     Dmitry Safonov <dsafonov@...tuozzo.com>
 AuthorDate: Wed, 23 Nov 2016 21:13:30 +0300
 Committer:  Ingo Molnar <mingo@...nel.org>
 CommitDate: Thu, 24 Nov 2016 06:01:05 +0100

 x86/coredump: Always use user_regs_struct for compat_elf_gregset_t

Thanks,

	Ingo