Messages in this thread | | | Subject | Re: [PATCH] x86/coredump: always use user_regs_struct for compat_elf_gregset_t | From | Dmitry Safonov <> | Date | Fri, 9 Dec 2016 14:29:55 +0300 |
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On 12/09/2016 02:14 AM, Andy Lutomirski wrote: > On Nov 23, 2016 10:16 AM, "Dmitry Safonov" <dsafonov@virtuozzo.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.
Hi Andy,
Here it is:
> static int fill_thread_core_info(struct elf_thread_core_info *t, > const struct user_regset_view *view, > long signr, size_t *total) > { > unsigned int i; > unsigned int regset_size = view->regsets[0].n * view->regsets[0].size;
For now the regset_size is 64-bit registers set's size if 32-bit ELF crashed with 64-bit CS.
> > /* > * NT_PRSTATUS is the one special case, because the regset data > * goes into the pr_reg field inside the note contents, rather > * than being the whole note contents. We fill the reset in here. > * We assume that regset 0 is NT_PRSTATUS. > */ > fill_prstatus(&t->prstatus, t->task, signr); > (void) view->regsets[0].get(t->task, &view->regsets[0], 0, regset_size, > &t->prstatus.pr_reg, NULL);
And here is writing to elf_thread_core_info::prstatus::pr_reg, prstatus member is typed compat_elf_prstatus as binfmt_elf interpreter that was used to load the program is from fs/compat_binfmt_elf.c: > #define elf_prstatus compat_elf_prstatus > #define elf_prpsinfo compat_elf_prpsinfo
So, we're overwriting elf_thread_core_info structure's content by writing bigger regset than it can hold. (.get() method is genregs_get() from arch/x86/kernel/ptrace.c)
The crash happens afterwards, when we're trying to dereference some fields of elf_thread_core_info - for me it was as you can see in writenote(): [<ffffffff811d6929>] ? writenote+0x19/0xa0 [<ffffffff811d9479>] elf_core_dump+0x11a9/0x1480 [<ffffffff811dc70b>] do_coredump+0xa6b/0xe60 [<ffffffff81065820>] ? signal_wake_up_state+0x20/0x30 [<ffffffff81065941>] ? complete_signal+0xf1/0x1f0 [<ffffffff810679e8>] get_signal+0x1a8/0x5c0 [<ffffffff8101b1a3>] do_signal+0x23/0x660
In my point of view 64-bit regset is generated rightly - otherwise I couldn't see x86_64 registers in gdb for that kind of crashes. So, I fixed it as simple as possible - by having one size for compat_elf_gregset_t independent of CONFIG_X86_X32_ABI option.
-- Dmitry
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