Re: [PATCH v3] usb: dwc2: host: Fix use after free w/ simultaneous irqs

[Doug Anderson <dianders@chromium.org>]

Hi,

On Fri, Oct 16, 2015 at 2:59 PM, Douglas Anderson <dianders@chromium.org> wrote:
> From: Doug Anderson <dianders@chromium.org>
>
> While plugging / unplugging on a DWC2 host port with "slub_debug=FZPUA"
> enabled, I found a crash that was quite obviously a use after free.
>
> It appears that in some cases when we handle the various sub-cases of
> HCINT we may end up freeing the QTD.  If there is more than one bit set
> in HCINT we may then end up continuing to use the QTD, which is bad.
> Let's be paranoid and check for this after each sub-case.  This should
> be safe since we officially have the "hsotg->lock" (it was grabbed in
> dwc2_handle_hcd_intr).
>
> The specific crash I found was:
>  Unable to handle kernel paging request at virtual address 6b6b6b9f
>
> At the time of the crash, the kernel reported:
>  (dwc2_hc_nak_intr+0x5c/0x198)
>  (dwc2_handle_hcd_intr+0xa84/0xbf8)
>  (_dwc2_hcd_irq+0x1c/0x20)
>  (usb_hcd_irq+0x34/0x48)
>
> Popping into kgdb found that "*qtd" was filled with "0x6b", AKA qtd had
> been freed and filled with slub_debug poison.
>
> kgdb gave a little better stack crawl:
>  0 dwc2_hc_nak_intr (hsotg=hsotg@entry=0xec42e058,
>      chan=chan@entry=0xec546dc0, chnum=chnum@entry=4,
>      qtd=qtd@entry=0xec679600) at drivers/usb/dwc2/hcd_intr.c:1237
>  1 dwc2_hc_n_intr (chnum=4, hsotg=0xec42e058) at
>      drivers/usb/dwc2/hcd_intr.c:2041
>  2 dwc2_hc_intr (hsotg=0xec42e058) at drivers/usb/dwc2/hcd_intr.c:2078
>  3 dwc2_handle_hcd_intr (hsotg=0xec42e058) at
>      drivers/usb/dwc2/hcd_intr.c:2128
>  4 _dwc2_hcd_irq (hcd=<optimized out>) at drivers/usb/dwc2/hcd.c:2837
>  5 usb_hcd_irq (irq=<optimized out>, __hcd=<optimized out>) at
>      drivers/usb/core/hcd.c:2353
>
> Popping up to frame #1 (dwc2_hc_n_intr) found:
>  (gdb) print /x hcint
>  $12 = 0x12
>
> AKA:
>  #define HCINTMSK_CHHLTD  (1 << 1)
>  #define HCINTMSK_NAK     (1 << 4)
>
> Further debugging found that by simulating receiving those two
> interrupts at the same time it was trivial to replicate the
> use-after-free.  See <http://crosreview.com/305712> for a patch and
> instructions.  This lead to getting the following stack crawl of the
> actual free:
>  0  arch_kgdb_breakpoint () at arch/arm/include/asm/outercache.h:103
>  1  kgdb_breakpoint () at kernel/debug/debug_core.c:1054
>  2  dwc2_hcd_qtd_unlink_and_free (hsotg=<optimized out>, qh=<optimized
>       out>, qtd=0xe4479a00) at drivers/usb/dwc2/hcd.h:488
>  3  dwc2_deactivate_qh (free_qtd=<optimized out>, qh=0xe5efa280,
>       hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:671
>  4  dwc2_release_channel (hsotg=hsotg@entry=0xed424618,
>       chan=chan@entry=0xed5be000, qtd=<optimized out>,
>       halt_status=<optimized out>) at drivers/usb/dwc2/hcd_intr.c:742
>  5  dwc2_halt_channel (hsotg=0xed424618, chan=0xed5be000, qtd=<optimized
>       out>, halt_status=<optimized out>) at
>       drivers/usb/dwc2/hcd_intr.c:804
>  6  dwc2_complete_non_periodic_xfer (chnum=<optimized out>,
>       halt_status=<optimized out>, qtd=<optimized out>, chan=<optimized
>       out>, hsotg=<optimized out>) at drivers/usb/dwc2/hcd_intr.c:889
>  7  dwc2_hc_xfercomp_intr (hsotg=hsotg@entry=0xed424618,
>       chan=chan@entry=0xed5be000, chnum=chnum@entry=6,
>       qtd=qtd@entry=0xe4479a00) at drivers/usb/dwc2/hcd_intr.c:1065
>  8  dwc2_hc_chhltd_intr_dma (qtd=0xe4479a00, chnum=6, chan=0xed5be000,
>       hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:1823
>  9  dwc2_hc_chhltd_intr (qtd=0xe4479a00, chnum=6, chan=0xed5be000,
>       hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:1944
>  10 dwc2_hc_n_intr (chnum=6, hsotg=0xed424618) at
>       drivers/usb/dwc2/hcd_intr.c:2052
>  11 dwc2_hc_intr (hsotg=0xed424618) at drivers/usb/dwc2/hcd_intr.c:2097
>  12 dwc2_handle_hcd_intr (hsotg=0xed424618) at
>       drivers/usb/dwc2/hcd_intr.c:2147
>  13 _dwc2_hcd_irq (hcd=<optimized out>) at drivers/usb/dwc2/hcd.c:2837
>  14 usb_hcd_irq (irq=<optimized out>, __hcd=<optimized out>) at
>       drivers/usb/core/hcd.c:2353
>
> Though we could add specific code to handle this case, adding the
> general purpose code to check for all cases where qtd might be freed
> seemed safer.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> Changes in v3:
> - Don't pass NULL if qtd freed, just return (John Youn)
> - Don't keep track of interrupts left: list_first_entry() is fast.
>
> Changes in v2:
> - Add static as correctly pointed by kbuild test robot
>
>  drivers/usb/dwc2/hcd_intr.c | 67 ++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 57 insertions(+), 10 deletions(-)

I just tested this latest patch together with the descriptor DMA
patches.  I didn't track down why, but something about that series
ended up making "qh" NULL after an interrupt.  I'll post up v4 that
fixes that.  Sorry for the noise.

-Doug