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SubjectRe: [qemu] boot failed: Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
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On 7/6/2020 5:53 AM, Arnd Bergmann wrote:
> On Mon, Jul 6, 2020 at 1:03 PM Naresh Kamboju <naresh.kamboju@linaro.org> wrote:
>>
>> While booting qemu_arm64 and qemu_arm with Linux version 5.8.0-rc3-next-20200706
>> the kernel panic noticed due to kernel NULL pointer dereference.
>>
>> metadata:
>> git branch: master
>> git repo: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
>> git commit: 5680d14d59bddc8bcbc5badf00dbbd4374858497
>> git describe: next-20200706
>> make_kernelversion: 5.8.0-rc3
>> kernel-config:
>> https://builds.tuxbuild.com/Glr-Ql1wbp3qN3cnHogyNA/kernel.config
>>
>> qemu arm64 boot crash log,
>>
>> [ 0.972053] Unable to handle kernel NULL pointer dereference at
>> virtual address 0000000000000000
>> [ 0.975301] Mem abort info:
>> [ 0.976316] ESR = 0x96000004
>> [ 0.977378] EC = 0x25: DABT (current EL), IL = 32 bits
>> [ 0.979363] SET = 0, FnV = 0
>> [ 0.980458] EA = 0, S1PTW = 0
>> [ 0.981583] Data abort info:
>> [ 0.982634] ISV = 0, ISS = 0x00000004
>> [ 0.984213] CM = 0, WnR = 0
>> [ 0.985260] [0000000000000000] user address but active_mm is swapper
>> [ 0.987600] Internal error: Oops: 96000004 [#1] PREEMPT SMP
>> [ 0.989557] Modules linked in:
>> [ 0.990671] CPU: 2 PID: 1 Comm: swapper/0 Not tainted
>> 5.8.0-rc3-next-20200706 #1
>> [ 0.993711] Hardware name: linux,dummy-virt (DT)
>> [ 0.995708] pstate: 00000005 (nzcv daif -PAN -UAO BTYPE=--)
>> [ 0.998168] pc : pl011_dma_probe+0x90/0x360
>
> This is the code from you vmlinux file:
>
> ffff8000107233e4: b90087e2 str w2, [sp, #132]
> ffff8000107233e8: 97fcf14c bl ffff80001065f918
> <dma_request_chan>
> ffff8000107233ec: aa0003f4 mov x20, x0
> ffff8000107233f0: b140041f cmn x0, #0x1, lsl #12
> ffff8000107233f4: 54000488 b.hi ffff800010723484
> <pl011_dma_probe+0x11c> // b.pmore
> ffff8000107233f8: f9400280 ldr x0, [x20]
> ffff8000107233fc: f9409c02 ldr x2, [x0, #312]
> ffff800010723400: b4000082 cbz x2, ffff800010723410
> <pl011_dma_probe+0xa8>
>
> It's the "ldr x0, [x20]" dereferencing 'chan' in pl011_dma_probe() after
> checking it for an error value. However it's a NULL pointer, not an
> error pointer, indicating that there is a bug in the dmaengine driver
> that you use here, or in the dmaengine core code.

Arnd,
I'm looking at the pl001_dma_probe(), I think we could make it more robust if it
uses IS_ERR_OR_NULL(chan) instead of IS_ERR(). Should I send a patch for it? I
suppose looking at the comment header for dma_request_chan() it does say return
chan ptr or error ptr. Sorry I missed that.


Vinod,
It looks like the only fix for dmaengine for the patch is where Arnd pointed out
as far as I can tell after auditing it. Let me know how you want to handle this.
Thanks!

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 0d6529eff66f..48e159e83cf5 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -852,7 +852,7 @@ struct dma_chan *dma_request_chan(struct device *dev, const
char *name)
mutex_lock(&dma_list_mutex);
if (list_empty(&dma_device_list)) {
mutex_unlock(&dma_list_mutex);
- return NULL;
+ return ERR_PTR(-ENODEV);
}

list_for_each_entry_safe(d, _d, &dma_device_list, global_node) {

>
> I don't see anything suspicious in dmaengine drivers, but there is a
> recent series
> from Dave Jiang that might explain it. Could you try reverting commit
> deb9541f5052 ("dmaengine: check device and channel list for empty")?
>
> I think the broken change is this one:
>
> @@ -819,6 +850,11 @@ struct dma_chan *dma_request_chan(struct device
> *dev, const char *name)
>
> /* Try to find the channel via the DMA filter map(s) */
> mutex_lock(&dma_list_mutex);
> + if (list_empty(&dma_device_list)) {
> + mutex_unlock(&dma_list_mutex);
> + return NULL;
> + }
> +
> list_for_each_entry_safe(d, _d, &dma_device_list, global_node) {
> dma_cap_mask_t mask;
> const struct dma_slave_map *map = dma_filter_match(d,
> name, dev);
>
> which needs to return an error code like -ENODEV instead of NULL. There
> may be other changes in the same patch that introduce the same bug
> elsewhere.
>
> Arnd
>

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