Re: [PATCH v2 0/4] staging: vchiq: Fix vchiq_read return value in case of error

[Nicolas Saenz Julienne <nsaenzjulienne@suse.de>]

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Hi Marcelo,

On Wed, 2019-11-20 at 15:20 -0500, Marcelo Diop-Gonzalez wrote:
> This is a proposed fix of an issue regarding the handling of positive
> return values from copy_to_user() in vchiq_read(). It looks like the
> struct dump_context passed to the vchiq_dump() function keeps track
> of the number of bytes written in the context->actual field. When
> copy_to_user() returns a positive value, this is set to -EFAULT. The
> problem is that this is never returned to the user, and instead the
> function continues, adding the number of bytes that should have
> been copied to context->actual.
> 
> Running the following program:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <errno.h>
> 
> int main(int argc, char **argv) {
>         int fd = open("/dev/vchiq", 0);
>         if (fd < 0)
>                 exit(-1);
>         sbrk(getpagesize());
>         char *bad_buf = sbrk(0)-100;
>         int x = read(fd, bad_buf, 2000);
>         printf("%d %d\n", x, errno);
>         puts(bad_buf);
> }
> 
> gives this output:
> 
> -1 1
> State 0: CONNECTED
>   tx_po
> 
> 
>   Remote: slots 2-32 tx_pos=578 recycle=1f
>     Slots claimed:
> 
> Note the mangled output and incorrect errno value. Messing with the
> constants in that toy program changes the results. Sometimes read()
> returns with no error, sometimes it returns with a wrong error code,
> sometimes with the right one. But it seems that it only ever returns an
> error at all accidentally, due to the fact that the comparison between
> context->actual and context->space in vchiq_dump() is unsigned, so that
> that function won't do anything else if it ever sets context->actual
> to a negative value.
> 
> After this patchset, the above program prints this:
> 
> -1 14
> State 0: CONNECTED
>   tx_pos=b4a218(@165de6b4), rx_pos=ae0668(@f02b54f4)
>   Version: 8 (min 3)
>   Stats
> 
> Help with testing would be appreciated. So far I've basically just
> diffed the output of 'cat /dev/vchiq', run the program above with
> a few different values, and run vchiq_test a few times.
> 
> These were applied to the staging-next branch of the tree
> at git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git

For the whole series:

Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

Thanks,
Nicolas


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