Re: Out-of-bounds write in driver_override_show

[Alex Williamson]

On Mon, 2014-12-01 at 23:20 +0400, Dmitry Vyukov wrote:
> On Mon, Dec 1, 2014 at 10:12 PM, Alex Williamson
> <alex.williamson@xxxxxxxxxx> wrote:
> > On Sat, 2014-11-29 at 10:05 -0800, Greg Kroah-Hartman wrote:
> >> On Sat, Nov 29, 2014 at 01:38:10PM +0400, Dmitry Vyukov wrote:
> >> > Hello,
> >> >
> >> > I am working on Kernel AddressSanitizer, a fast memory error detector
> >> > for kernel:
> >> > https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
> >> >
> >> > Here is an error report that I got while running trinity:
> >>
> >> Alex, this looks like the code you added, you need to properly check for
> >> the size of the string before writing it back...
> >
> > Thanks for the report, I'm not sure I entirely understand the issue
> > though.  The string at pci_dev.driver_override is bounds checked when
> > set in driver_override_store() and I think that the buffer for a sysfs
> > show is always a page, right?  So by checking strlen are we just
> > guarding against some in-kernel setting of driver_override other than
> > the _store() path?  Thanks,
> 
> 
> Hi!
> 
> driver_override_store checks it against PATH_MAX, and PATH_MAX is
> generally unrelated to PAGE_SIZE. It happens so that in my config
> PATH_MAX==PAGE_SIZE. Yet, the sprintf writes PAGE_SIZE+1 (+1 for the
> trailing 0 terminator). So even if PATH_MAX==PAGE_SIZE it smashes the
> first byte of the next memory block.

Ok, so we just need to use snprintf(buf, PAGE_SIZE,...) to avoid the
smashing, assumptions about PATH_MAX vs PAGE_SIZE, and check anything in
the kernel that may set driver_override.  We might end-up dropping the
newline on the _show(), but I don't know that it's worth blurring
accepting a PATH_MAX string to resolve that.  I'll post a patch.
Thanks,

Alex

> >> > BUG: AddressSanitizer: out of bounds access in vsnprintf+0xd0/0x890 at
> >> > addr ffff880057483261
> >> > Write of size 1 by task trinity-c54/4461
> >> > =============================================================================
> >> > BUG kmalloc-4096 (Tainted: G        W     ): kasan error
> >> > -----------------------------------------------------------------------------
> >> > INFO: Allocated in seq_open+0x5a/0xe0 age=1571449 cpu=0 pid=28869
> >> > __slab_alloc+0x4c4/0x4e0
> >> > __kmalloc+0x18b/0x1b0
> >> > seq_buf_alloc+0x16/0x40
> >> > traverse+0x243/0x350
> >> > seq_read+0x434/0x6b0
> >> > kernfs_fop_read+0x176/0x1f0
> >> > vfs_read+0xd7/0x240
> >> > SyS_read+0x57/0xc0
> >> > system_call_fastpath+0x12/0x17
> >> > INFO: Slab 0xffffea00015d2000 objects=7 used=7 fp=0x          (null)
> >> > flags=0x100000000004080
> >> > INFO: Object 0xffff880057482260 @offset=8800 fp=0xffffffffffffffff
> >> >
> >> > CPU: 0 PID: 4461 Comm: trinity-c54 Tainted: G    B   W      3.18.0-rc1+ #8
> >> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> >> >  ffffffff81fbdaac ffff88008e3ab8c8 ffffffff81c85c3f 0000000000001130
> >> >  ffff88011b003200 ffff88008e3ab8f8 ffffffff811ee018 ffff88011b003200
> >> >  ffffea00015d2000 ffff880057482260 ffff8800d748225f ffff88008e3ab928
> >> > Call Trace:
> >> >  [<ffffffff811f76a5>] __asan_store1+0x75/0xb0 mm/kasan/kasan.c:417
> >> >  [<ffffffff8147b660>] vsnprintf+0xd0/0x890 lib/vsprintf.c:1860
> >> >  [<ffffffff8147bf70>] sprintf+0x40/0x50 lib/vsprintf.c:1989
> >> >  [<ffffffff816e1e51>] driver_override_show+0x31/0x40 drivers/base/platform.c:755
> >> >  [<ffffffff816d92d9>] dev_attr_show+0x39/0x80 drivers/base/core.c:120
> >> >  [<ffffffff812a3892>] sysfs_kf_seq_show+0x152/0x230 fs/sysfs/file.c:63
> >> >  [<ffffffff812a13a6>] kernfs_seq_show+0x76/0x90 fs/kernfs/file.c:168
> >> >  [<ffffffff8122f755>] traverse+0x185/0x350 fs/seq_file.c:120
> >> >  [<ffffffff8122fee4>] seq_read+0x434/0x6b0 fs/seq_file.c:191
> >> > [<ffffffff812a22f6>] kernfs_fop_read+0x176/0x1f0 fs/kernfs/file.c:244
> >> >  [<ffffffff811fc131>] do_loop_readv_writev+0x71/0xa0 fs/read_write.c:708
> >> >  [<ffffffff811fe850>] do_readv_writev+0x350/0x360 fs/read_write.c:842
> >> >  [<ffffffff811fe8ab>] vfs_readv+0x4b/0x70 fs/read_write.c:867
> >> >  [<     inlined    >] SyS_preadv+0xca/0xf0 SYSC_preadv fs/read_write.c:945
> >> >  [<ffffffff811fec0a>] SyS_preadv+0xca/0xf0 fs/read_write.c:931
> >> >  [<ffffffff81c91f69>] system_call_fastpath+0x12/0x17
> >> > arch/x86/kernel/entry_64.S:422
> >> > Memory state around the buggy address:
> >> >  ffff880057482f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> >  ffff880057483000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> >  ffff880057483080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> >  ffff880057483100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> >  ffff880057483180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> > >ffff880057483200: 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc fc
> >> >                                                        ^
> >> >  ffff880057483280: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >> >  ffff880057483300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >> >  ffff880057483380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> >  ffff880057483400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> >  ffff880057483480: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >> > ==================================================================
> >> > dev_attr_show: driver_override_show+0x0/0x40 returned bad count
> >> > fill_read_buffer: dev_attr_show+0x0/0x80 returned bad count
> >> >
> >> >
> >> > My source is on revision f114040e3ea6e07372334ade75d1ee0775c355e1.
> >> >
> >> > I've looked at source code and it seems that driver_override_store
> >> > does not do length sanitization, so driver_override_show smashes
> >> > subsequent memory blocks in sprinf (with user-provided data?).
> >
> >
> >



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