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Date:   Sat, 09 May 2020 10:58:11 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Kyungtae Kim <kt0755@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     syzkaller <syzkaller@...glegroups.com>,
        USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Tian <dave.jing.tian@...il.com>
Subject: Re: [PATCH] USB: gadget: fix illegal array access in binding with UDC

Kyungtae Kim <kt0755@...il.com> writes:

> FuzzUSB (a variant of syzkaller) found an illegal array access
> using an incorrect index while binding a gadget with UDC.
>
> Reference: https://www.spinics.net/lists/linux-usb/msg194331.html
>
> This bug occurs when a size variable used for a buffer
> is misused to access its strcpy-ed buffer.
> Given a buffer along with its size variable (taken from user input),
> from which, a new buffer is created using kstrdup().
> Due to the original buffer containing 0 value in the middle,
> the size of the kstrdup-ed buffer becomes smaller than that of the original.
> So accessing the kstrdup-ed buffer with the same size variable
> triggers memory access violation.
>
> The fix makes sure no zero value in the buffer,
> by comparing the strlen() of the orignal buffer with the size variable,
> so that the access to the kstrdup-ed buffer is safe.
>
>
> BUG: KASAN: slab-out-of-bounds in gadget_dev_desc_UDC_store+0x1ba/0x200
> drivers/usb/gadget/configfs.c:266
> Read of size 1 at addr ffff88806a55dd7e by task syz-executor.0/17208
>
> CPU: 2 PID: 17208 Comm: syz-executor.0 Not tainted 5.6.8 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0xce/0x128 lib/dump_stack.c:118
>  print_address_description.constprop.4+0x21/0x3c0 mm/kasan/report.c:374
>  __kasan_report+0x131/0x1b0 mm/kasan/report.c:506
>  kasan_report+0x12/0x20 mm/kasan/common.c:641
>  __asan_report_load1_noabort+0x14/0x20 mm/kasan/generic_report.c:132
>  gadget_dev_desc_UDC_store+0x1ba/0x200 drivers/usb/gadget/configfs.c:266
>  flush_write_buffer fs/configfs/file.c:251 [inline]
>  configfs_write_file+0x2f1/0x4c0 fs/configfs/file.c:283
>  __vfs_write+0x85/0x110 fs/read_write.c:494
>  vfs_write+0x1cd/0x510 fs/read_write.c:558
>  ksys_write+0x18a/0x220 fs/read_write.c:611
>  __do_sys_write fs/read_write.c:623 [inline]
>  __se_sys_write fs/read_write.c:620 [inline]
>  __x64_sys_write+0x73/0xb0 fs/read_write.c:620
>  do_syscall_64+0x9e/0x510 arch/x86/entry/common.c:294
>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
>
> Signed-off-by: Kyungtae Kim <kt0755@...il.com>
> Reported-and-tested-by: Kyungtae Kim <kt0755@...il.com>
>
> ---
>  drivers/usb/gadget/configfs.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> index 32b637e3e1fa..6a9aa4413d64 100644
> --- a/drivers/usb/gadget/configfs.c
> +++ b/drivers/usb/gadget/configfs.c
> @@ -260,6 +260,9 @@ static ssize_t gadget_dev_desc_UDC_store(struct
> config_item *item,

patch is line-wrapped, please fix and resend.

-- 
balbi

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