Linux Virtualization - Dec 2013 - [PATCH] 9p/trans_virtio.c: Fix broken zero-copy on vmalloc() buffers

The 9p-virtio transport does zero copy on things larger than 1024 bytes
in size. It accomplishes this by returning the physical addresses of
pages to the virtio-pci device. At present, the translation is usually a
bit shift.

However, that approach produces an invalid page address when we
read/write to vmalloc buffers, such as those used for Linux kernle
modules. This causes QEMU to die printing:

qemu-system-x86_64: virtio: trying to map MMIO memory

This patch enables 9p-virtio to correctly handle this case. This not
only enables us to load Linux kernel modules off virtfs, but also
enables ZFS file-based vdevs on virtfs to be used without killing QEMU.

Also, special thanks to both Avi Kivity and Alexander Graf for their
interpretation of QEMU backtraces. Without their guidence, tracking down
this bug would have taken much longer.

Signed-off-by: Richard Yao <ryao at gentoo.org>
---
 net/9p/trans_virtio.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 9c5a1aa..5d1d04b 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -340,7 +340,10 @@ static int p9_get_mapped_pages(struct virtio_chan *chan,
 		int count = nr_pages;
 		while (nr_pages) {
 			s = rest_of_page(data);
-			pages[index++] = kmap_to_page(data);
+			if (is_vmalloc_or_module_addr(data))
+				pages[index++] = vmalloc_to_page(data);
+			else
+				pages[index++] = kmap_to_page(data);
 			data += s;
 			nr_pages--;
 		}
-- 
1.8.3.2

On 04.12.2013, at 21:43, Richard Yao <ryao at gentoo.org> wrote:
> The 9p-virtio transport does zero copy on things larger than 1024 bytes
> in size. It accomplishes this by returning the physical addresses of
> pages to the virtio-pci device. At present, the translation is usually a
> bit shift.
> 
> However, that approach produces an invalid page address when we
> read/write to vmalloc buffers, such as those used for Linux kernle
> modules. This causes QEMU to die printing:
> 
> qemu-system-x86_64: virtio: trying to map MMIO memory
> 
> This patch enables 9p-virtio to correctly handle this case. This not
> only enables us to load Linux kernel modules off virtfs, but also
> enables ZFS file-based vdevs on virtfs to be used without killing QEMU.
> 
> Also, special thanks to both Avi Kivity and Alexander Graf for their
> interpretation of QEMU backtraces. Without their guidence, tracking down
> this bug would have taken much longer.
> 
> Signed-off-by: Richard Yao <ryao at gentoo.org>
Looks good and clean to me, but I'm not expert on the mm subsystem.

Acked-by: Alexander Graf <agraf at suse.de>


Alex

On Wed, Dec 04, 2013 at 08:43:18PM +0000, Richard Yao
wrote:
> The 9p-virtio transport does zero copy on things larger than 1024 bytes
> in size. It accomplishes this by returning the physical addresses of
> pages to the virtio-pci device. At present, the translation is usually a
> bit shift.
> 
> However, that approach produces an invalid page address when we
> read/write to vmalloc buffers, such as those used for Linux kernle
> modules. This causes QEMU to die printing:
> 
> qemu-system-x86_64: virtio: trying to map MMIO memory
> 
> This patch enables 9p-virtio to correctly handle this case. This not
> only enables us to load Linux kernel modules off virtfs, but also
> enables ZFS file-based vdevs on virtfs to be used without killing QEMU.
> 
> Also, special thanks to both Avi Kivity and Alexander Graf for their
> interpretation of QEMU backtraces. Without their guidence, tracking down
> this bug would have taken much longer.
> 
> Signed-off-by: Richard Yao <ryao at gentoo.org>
> ---
>  net/9p/trans_virtio.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 9c5a1aa..5d1d04b 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -340,7 +340,10 @@ static int p9_get_mapped_pages(struct virtio_chan
*chan,
>  		int count = nr_pages;
>  		while (nr_pages) {
>  			s = rest_of_page(data);
> -			pages[index++] = kmap_to_page(data);
> +			if (is_vmalloc_or_module_addr(data))
Can this really end up being a module address?

Will

On 12/06/2013 06:14 AM, Will Deacon wrote:
> On Wed, Dec 04, 2013 at 08:43:18PM +0000, Richard Yao wrote:
>> The 9p-virtio transport does zero copy on things larger than 1024 bytes
>> in size. It accomplishes this by returning the physical addresses of
>> pages to the virtio-pci device. At present, the translation is usually
a
>> bit shift.
>>
>> However, that approach produces an invalid page address when we
>> read/write to vmalloc buffers, such as those used for Linux kernle
>> modules. This causes QEMU to die printing:
>>
>> qemu-system-x86_64: virtio: trying to map MMIO memory
>>
>> This patch enables 9p-virtio to correctly handle this case. This not
>> only enables us to load Linux kernel modules off virtfs, but also
>> enables ZFS file-based vdevs on virtfs to be used without killing QEMU.
>>
>> Also, special thanks to both Avi Kivity and Alexander Graf for their
>> interpretation of QEMU backtraces. Without their guidence, tracking
down
>> this bug would have taken much longer.
>>
>> Signed-off-by: Richard Yao <ryao at gentoo.org>
>> ---
>>  net/9p/trans_virtio.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
>> index 9c5a1aa..5d1d04b 100644
>> --- a/net/9p/trans_virtio.c
>> +++ b/net/9p/trans_virtio.c
>> @@ -340,7 +340,10 @@ static int p9_get_mapped_pages(struct virtio_chan
*chan,
>>  		int count = nr_pages;
>>  		while (nr_pages) {
>>  			s = rest_of_page(data);
>> -			pages[index++] = kmap_to_page(data);
>> +			if (is_vmalloc_or_module_addr(data))
> 
> Can this really end up being a module address?
Yes. Here is the stacktrace to prove it:

[<ffffffff814878ce>] p9_virtio_zc_request+0x45e/0x510
[<ffffffff814814ed>] p9_client_zc_rpc.constprop.16+0xfd/0x4f0
[<ffffffff814839dd>] p9_client_read+0x15d/0x240
[<ffffffff811c8440>] v9fs_fid_readn+0x50/0xa0
[<ffffffff811c84a0>] v9fs_file_readn+0x10/0x20
[<ffffffff811c84e7>] v9fs_file_read+0x37/0x70
[<ffffffff8114e3fb>] vfs_read+0x9b/0x160
[<ffffffff81153571>] kernel_read+0x41/0x60
[<ffffffff810c83ab>] copy_module_from_fd.isra.34+0xfb/0x180
[<ffffffff810cc420>] SyS_finit_module+0x70/0xd0
[<ffffffff814a08fd>] system_call_fastpath+0x1a/0x1f
[<ffffffffffffffff>] 0xffffffffffffffff

This is easily reproducible by trying to load a module off virtfs. QEMU
will print the message that I cited in the commit message and then kill
itself.

P.S. I omitted the CC list the first time I sent this, so I am resending
with the correct CC list.

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