Re: [PATCH 7/7] firmware: firmware: fix NULL pointer dereference in __fw_load_abort()

[Greg KH]

On Mon, Jan 23, 2017 at 08:11:11AM -0800, Luis R. Rodriguez wrote:
> Since commit 5d47ec02c37ea632398cb251c884e3a488dff794
> ("firmware: Correct handling of fw_state_wait() return value")
> fw_load_abort(fw_priv) could be called twice and lead us to a
> kernel crash. This happens only when the firmware fallback mechanism
> (regular or custom) is used. The fallback mechanism exposes a sysfs
> interface for userspace to upload a file and notify the kernel when
> the file is loaded and ready, or to cancel an upload by echo'ing -1
> into on the loading file:
> 
> echo -n "-1" > /sys/$DEVPATH/loading
> 
> This will call fw_load_abort(). Some distributions actually have
> a udev rule in place to *always* immediately cancel all firmware
> fallback mechanism requests (Debian, OpenSUSE), they have:
> 
>   $ cat /lib/udev/rules.d/50-firmware.rules
>   # stub for immediately telling the kernel that userspace firmware loading
>   # failed; necessary to avoid long timeouts with CONFIG_FW_LOADER_USER_HELPER=y
>   SUBSYSTEM=="firmware", ACTION=="add", ATTR{loading}="-1
> 
> This was done since udev removed the firmware fallback mechanism a while ago
> and a long standing misunderstood issues with the timeout (but now corrected).
> Distributions with this udev rule would run into this crash only if the
> fallback mechanism is used. Since most distributions disable by default
> using the fallback mechanism (CONFIG_FW_LOADER_USER_HELPER_FALLBACK), this
> would typicaly mean only 2 drivers which *require* the fallback mechanism
> could typically incur a crash: drivers/firmware/dell_rbu.c and the
> drivers/leds/leds-lp55xx-common.c driver.
> 
> Distributions enabling CONFIG_FW_LOADER_USER_HELPER_FALLBACK are clearly
> more exposed as every file not found through a firmware request will
> use the fallback mechanism.
> 
> The crash happens because after commit 5b029624948d ("firmware: do not
> use fw_lock for fw_state protection") and subsequent fix commit
> 5d47ec02c37ea6 ("firmware: Correct handling of fw_state_wait() return
> value") a race can happen between this cancelation and the firmware
> fw_state_wait_timeout() being woken up after a state change with which
> fw_load_abort() as that calls swake_up(). Upon error fw_state_wait_timeout()
> will also again call fw_load_abort() and trigger a null reference.
> 
> At first glance we could just fix this with a !buf check on
> fw_load_abort() before accessing buf->fw_st, however there is
> a logical issue in having a state machine used for the fallback
> mechanism and preventing access from it once we abort as its inside
> the buf (buf->fw_st).
> 
> The firmware_class.c code is setting the buf to NULL to annotate an
> abort has occurred. Replace this mechanism by simply using the state check
> instead. All the other code in place already uses similar checks
> for aborting as well so no further changes are needed.
> 
> An oops can be reproduced with the new fw_fallback.sh fallback
> mechanism cancellation test. Either cancelling the fallback mechanism
> or the custom fallback mechanism triggers a crash.
> 
> mcgrof@piggy ~/linux-next/tools/testing/selftests/firmware
> (git::20170111-fw-fixes)$ sudo ./fw_fallback.sh
> 
> ./fw_fallback.sh: timeout works
> ./fw_fallback.sh: firmware comparison works
> ./fw_fallback.sh: fallback mechanism works
> 
> [ this then sits here when it is trying the cancellation test ]
> 
> Kernel log:
> 
> [   36.750521] test_firmware: loading 'nope-test-firmware.bin'
> [   36.751144] misc test_firmware: Direct firmware load for nope-test-firmware.bin failed with error -2
> [   36.752034] misc test_firmware: Falling back to user helper
> [   36.853324] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
> [   36.854221] IP: _request_firmware+0xa27/0xad0
> [   36.854671] PGD 0
> [   36.854672]
> [   36.855081] Oops: 0000 [#1] SMP
> [   36.855433] Modules linked in: test_firmware(E) ... etc ...
> [   36.857802] CPU: 1 PID: 1396 Comm: fw_fallback.sh Tainted: G        W E   4.10.0-rc3-next-20170111+ #30
> [   36.857802] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.1-0-g8891697-prebuilt.qemu-project.org 04/01/2014
> [   36.857802] task: ffff9740b27f4340 task.stack: ffffbb15c0bc8000
> [   36.857802] RIP: 0010:_request_firmware+0xa27/0xad0
> [   36.857802] RSP: 0018:ffffbb15c0bcbd10 EFLAGS: 00010246
> [   36.857802] RAX: 00000000fffffffe RBX: ffff9740afe5aa80 RCX: 0000000000000000
> [   36.857802] RDX: ffff9740b27f4340 RSI: 0000000000000283 RDI: 0000000000000000
> [   36.857802] RBP: ffffbb15c0bcbd90 R08: ffffbb15c0bcbcd8 R09: 0000000000000000
> [   36.857802] R10: 0000000894a0d4b1 R11: 000000000000008c R12: ffffffffc0312480
> [   36.857802] R13: 0000000000000005 R14: ffff9740b1c32400 R15: 00000000000003e8
> [   36.857802] FS:  00007f8604422700(0000) GS:ffff9740bfc80000(0000) knlGS:0000000000000000
> [   36.857802] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   36.857802] CR2: 0000000000000038 CR3: 000000012164c000 CR4: 00000000000006e0
> [   36.857802] Call Trace:
> [   36.857802]  request_firmware+0x37/0x50
> [   36.857802]  trigger_request_store+0x79/0xd0 [test_firmware]
> [   36.857802]  dev_attr_store+0x18/0x30
> [   36.857802]  sysfs_kf_write+0x37/0x40
> [   36.857802]  kernfs_fop_write+0x110/0x1a0
> [   36.857802]  __vfs_write+0x37/0x160
> [   36.857802]  ? _cond_resched+0x1a/0x50
> [   36.857802]  vfs_write+0xb5/0x1a0
> [   36.857802]  SyS_write+0x55/0xc0
> [   36.857802]  ? trace_do_page_fault+0x37/0xd0
> [   36.857802]  entry_SYSCALL_64_fastpath+0x1e/0xad
> [   36.857802] RIP: 0033:0x7f8603f49620
> [   36.857802] RSP: 002b:00007fff6287b788 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> [   36.857802] RAX: ffffffffffffffda RBX: 000055c307b110a0 RCX: 00007f8603f49620
> [   36.857802] RDX: 0000000000000016 RSI: 000055c3084d8a90 RDI: 0000000000000001
> [   36.857802] RBP: 0000000000000016 R08: 000000000000c0ff R09: 000055c3084d6336
> [   36.857802] R10: 000055c307b108b0 R11: 0000000000000246 R12: 000055c307b13c80
> [   36.857802] R13: 000055c3084d6320 R14: 0000000000000000 R15: 00007fff6287b950
> [   36.857802] Code: 9f 64 84 e8 9c 61 fe ff b8 f4 ff ff ff e9 6b f9 ff
> ff 48 c7 c7 40 6b 8d 84 89 45 a8 e8 43 84 18 00 49 8b be 00 03 00 00 8b
> 45 a8 <83> 7f 38 02 74 08 e8 6e ec ff ff 8b 45 a8 49 c7 86 00 03 00 00
> [   36.857802] RIP: _request_firmware+0xa27/0xad0 RSP: ffffbb15c0bcbd10
> [   36.857802] CR2: 0000000000000038
> [   36.872685] ---[ end trace 6d94ac339c133e6f ]---
> 
> In above case the call hierarchy that causes the crash looks as follows:
> 
> lib/test_firmware.c request_firmware()
> -> fw_load_from_user_helper()
> -> _request_firmware_load()
> -> call fw_state_wait_timeout()
> 
> Some time later firmware_loading_store() scans a control value of "-1"
> -> switch(loading) case -1: will call
> -> fw_load_abort(fw_priv) which calls
> -> __fw_load_abort(fw_priv->buf)
> -> and set fw_priv->buf = NULL;
> 
> Upon being woken up via swake_up(), back in _request_firmware_load()
> fw_state_wait_timeout() returns -ENOENT
> -> since mentioned commit
> -> fw_load_abort(fw_priv) is called a second time
> -> and this time it would call:
> -> __fw_load_abort(NULL /* fw_priv->buf */)
> -> and we get: NULL->fw_st.status
> 
> Fixes: 5d47ec02c37e ("firmware: Correct handling of fw_state_wait() return value")
> Reported-and-Tested-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
> Reported-and-Tested-by: Patrick Bruenn <p.bruenn@xxxxxxxxxxxx>
> Reported-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> CC: <stable@xxxxxxxxxxxxxxx>    [3.10+]
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> ---
>  drivers/base/firmware_class.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)

Why is this patch 7/7?  Shouldn't it go into 4.10-final now?  Why wait
for 4.11-rc1?

thanks,

greg k-h