[PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

[PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

[Greg Kurz]

If it was interrupted by a signal, the 9p client may need to send some
more requests to the server for cleanup before returning to userspace.

To avoid such a last minute request to be interrupted right away, the
client memorizes if a signal is pending, clear TIF_SIGPENDING, handle
the request and call recalc_sigpending() before returning.

Unfortunately, if the transmission of this cleanup request fails for any
reason, the transport returns an error and the client propagates it right
away, without calling recalc_sigpending().

This ends up with -ERESTARTSYS from the initially interrupted request
crawling up to syscall exit, with TIF_SIGPENDING cleared by the cleanup
request. The specific signal handling code, which is responsible for
converting -ERESTARTSYS to -EINTR is not called, and userspace receives
the confusing errno value:

open: Unknown error 512 (512)

This is really hard to hit in real life. I discovered the issue while
working on hot-unplug of a virtio-9p-pci device with an instrumented
QEMU allowing to control request completion.

Both p9_client_zc_rpc() and p9_client_rpc() functions have this buggy
error path actually. Their code flow is a bit obscure and the best
thing to do would probably be a full rewrite: to really ensure this
situation of clearing TIF_SIGPENDING and returning -ERESTARTSYS can
never happen.

But given the general lack of interest for the 9p code, I won't risk
breaking more things. So this patch simply fix the buggy paths in both
functions with a trivial label+goto.

Thanks to Laurent Dufour for his help and suggestions on how to find
the root cause and how to fix it.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 net/9p/client.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index 4c8cf9c1631a..5154eaf19fff 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -769,7 +769,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 	if (err < 0) {
 		if (err != -ERESTARTSYS && err != -EFAULT)
 			c->status = Disconnected;
-		goto reterr;
+		goto recalc_sigpending;
 	}
 again:
 	/* Wait for the response */
@@ -804,6 +804,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 		if (req->status == REQ_STATUS_RCVD)
 			err = 0;
 	}
+recalc_sigpending:
 	if (sigpending) {
 		spin_lock_irqsave(&current->sighand->siglock, flags);
 		recalc_sigpending();
@@ -867,7 +868,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 		if (err == -EIO)
 			c->status = Disconnected;
 		if (err != -ERESTARTSYS)
-			goto reterr;
+			goto recalc_sigpending;
 	}
 	if (req->status == REQ_STATUS_ERROR) {
 		p9_debug(P9_DEBUG_ERROR, "req_status error %d\n", req->t_err);
@@ -885,6 +886,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 		if (req->status == REQ_STATUS_RCVD)
 			err = 0;
 	}
+recalc_sigpending:
 	if (sigpending) {
 		spin_lock_irqsave(&current->sighand->siglock, flags);
 		recalc_sigpending();

Re: [V9fs-developer] [PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

[Greg Kurz]

Hi Al,

It's been two years without any sign of life from 9p maintainers... :-\

Would you apply (or nack) this patch ?

Thanks,

--
Greg

PS: in the case you apply it, probable Cc stable@vger.kernel.org as well


On Thu, 08 Feb 2018 18:38:49 +0100

Greg Kurz <groug@kaod.org> wrote:

> If it was interrupted by a signal, the 9p client may need to send some
> more requests to the server for cleanup before returning to userspace.
> 
> To avoid such a last minute request to be interrupted right away, the
> client memorizes if a signal is pending, clear TIF_SIGPENDING, handle
> the request and call recalc_sigpending() before returning.
> 
> Unfortunately, if the transmission of this cleanup request fails for any
> reason, the transport returns an error and the client propagates it right
> away, without calling recalc_sigpending().
> 
> This ends up with -ERESTARTSYS from the initially interrupted request
> crawling up to syscall exit, with TIF_SIGPENDING cleared by the cleanup
> request. The specific signal handling code, which is responsible for
> converting -ERESTARTSYS to -EINTR is not called, and userspace receives
> the confusing errno value:
> 
> open: Unknown error 512 (512)
> 
> This is really hard to hit in real life. I discovered the issue while
> working on hot-unplug of a virtio-9p-pci device with an instrumented
> QEMU allowing to control request completion.
> 
> Both p9_client_zc_rpc() and p9_client_rpc() functions have this buggy
> error path actually. Their code flow is a bit obscure and the best
> thing to do would probably be a full rewrite: to really ensure this
> situation of clearing TIF_SIGPENDING and returning -ERESTARTSYS can
> never happen.
> 
> But given the general lack of interest for the 9p code, I won't risk
> breaking more things. So this patch simply fix the buggy paths in both
> functions with a trivial label+goto.
> 
> Thanks to Laurent Dufour for his help and suggestions on how to find
> the root cause and how to fix it.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  net/9p/client.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 4c8cf9c1631a..5154eaf19fff 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -769,7 +769,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>  	if (err < 0) {
>  		if (err != -ERESTARTSYS && err != -EFAULT)
>  			c->status = Disconnected;
> -		goto reterr;
> +		goto recalc_sigpending;
>  	}
>  again:
>  	/* Wait for the response */
> @@ -804,6 +804,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>  		if (req->status == REQ_STATUS_RCVD)
>  			err = 0;
>  	}
> +recalc_sigpending:
>  	if (sigpending) {
>  		spin_lock_irqsave(&current->sighand->siglock, flags);
>  		recalc_sigpending();
> @@ -867,7 +868,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
>  		if (err == -EIO)
>  			c->status = Disconnected;
>  		if (err != -ERESTARTSYS)
> -			goto reterr;
> +			goto recalc_sigpending;
>  	}
>  	if (req->status == REQ_STATUS_ERROR) {
>  		p9_debug(P9_DEBUG_ERROR, "req_status error %d\n", req->t_err);
> @@ -885,6 +886,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
>  		if (req->status == REQ_STATUS_RCVD)
>  			err = 0;
>  	}
> +recalc_sigpending:
>  	if (sigpending) {
>  		spin_lock_irqsave(&current->sighand->siglock, flags);
>  		recalc_sigpending();
> 
> 
> ------------------------------------------------------------------------------
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> _______________________________________________
> V9fs-developer mailing list
> V9fs-developer@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/v9fs-developer

Re: [V9fs-developer] [PATCH] net/9p: avoid -ERESTARTSYS leak to userspace

[jiangyiwen]

Hi Al,

I totally agree the Greg's suggestion, I think v9fs is the direction
as the VirtFS in the virtualization field, that it still deserves to
be used and developed, so I also suggestion you can apply (or nack)
the patch as v9fs maintainer, I hope you won't refuse.

Thanks,
Yiwen.

On 2018/2/21 0:48, Greg Kurz wrote:
> Hi Al,
> 
> It's been two years without any sign of life from 9p maintainers... :-\
> 
> Would you apply (or nack) this patch ?
> 
> Thanks,
> 
> --
> Greg
> 
> PS: in the case you apply it, probable Cc stable@vger.kernel.org as well
> 
> 
> On Thu, 08 Feb 2018 18:38:49 +0100
> 
> Greg Kurz <groug@kaod.org> wrote:
> 
>> If it was interrupted by a signal, the 9p client may need to send some
>> more requests to the server for cleanup before returning to userspace.
>>
>> To avoid such a last minute request to be interrupted right away, the
>> client memorizes if a signal is pending, clear TIF_SIGPENDING, handle
>> the request and call recalc_sigpending() before returning.
>>
>> Unfortunately, if the transmission of this cleanup request fails for any
>> reason, the transport returns an error and the client propagates it right
>> away, without calling recalc_sigpending().
>>
>> This ends up with -ERESTARTSYS from the initially interrupted request
>> crawling up to syscall exit, with TIF_SIGPENDING cleared by the cleanup
>> request. The specific signal handling code, which is responsible for
>> converting -ERESTARTSYS to -EINTR is not called, and userspace receives
>> the confusing errno value:
>>
>> open: Unknown error 512 (512)
>>
>> This is really hard to hit in real life. I discovered the issue while
>> working on hot-unplug of a virtio-9p-pci device with an instrumented
>> QEMU allowing to control request completion.
>>
>> Both p9_client_zc_rpc() and p9_client_rpc() functions have this buggy
>> error path actually. Their code flow is a bit obscure and the best
>> thing to do would probably be a full rewrite: to really ensure this
>> situation of clearing TIF_SIGPENDING and returning -ERESTARTSYS can
>> never happen.
>>
>> But given the general lack of interest for the 9p code, I won't risk
>> breaking more things. So this patch simply fix the buggy paths in both
>> functions with a trivial label+goto.
>>
>> Thanks to Laurent Dufour for his help and suggestions on how to find
>> the root cause and how to fix it.
>>
>> Signed-off-by: Greg Kurz <groug@kaod.org>
>> ---
>>  net/9p/client.c |    6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/9p/client.c b/net/9p/client.c
>> index 4c8cf9c1631a..5154eaf19fff 100644
>> --- a/net/9p/client.c
>> +++ b/net/9p/client.c
>> @@ -769,7 +769,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>>  	if (err < 0) {
>>  		if (err != -ERESTARTSYS && err != -EFAULT)
>>  			c->status = Disconnected;
>> -		goto reterr;
>> +		goto recalc_sigpending;
>>  	}
>>  again:
>>  	/* Wait for the response */
>> @@ -804,6 +804,7 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
>>  		if (req->status == REQ_STATUS_RCVD)
>>  			err = 0;
>>  	}
>> +recalc_sigpending:
>>  	if (sigpending) {
>>  		spin_lock_irqsave(&current->sighand->siglock, flags);
>>  		recalc_sigpending();
>> @@ -867,7 +868,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
>>  		if (err == -EIO)
>>  			c->status = Disconnected;
>>  		if (err != -ERESTARTSYS)
>> -			goto reterr;
>> +			goto recalc_sigpending;
>>  	}
>>  	if (req->status == REQ_STATUS_ERROR) {
>>  		p9_debug(P9_DEBUG_ERROR, "req_status error %d\n", req->t_err);
>> @@ -885,6 +886,7 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
>>  		if (req->status == REQ_STATUS_RCVD)
>>  			err = 0;
>>  	}
>> +recalc_sigpending:
>>  	if (sigpending) {
>>  		spin_lock_irqsave(&current->sighand->siglock, flags);
>>  		recalc_sigpending();
>>
>>
>> ------------------------------------------------------------------------------
>> Check out the vibrant tech community on one of the world's most
>> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
>> _______________________________________________
>> V9fs-developer mailing list
>> V9fs-developer@lists.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/v9fs-developer
> 
> 
> ------------------------------------------------------------------------------
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> _______________________________________________
> V9fs-developer mailing list
> V9fs-developer@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/v9fs-developer
> 
> .
>