[PATCH 0/3] staging: vchiq: Fix vchiq_read return value in case of error

Marcelo Diop-Gonzalez marcgonzalez at google.com
Wed Nov 20 15:36:45 UTC 2019

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)
	char *bad_buf = sbrk(0)-100;
	int x = read(fd, bad_buf, 2000);
	printf("%d %d\n", x, errno);

gives this output:

-1 1
  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
  tx_pos=b4a218(@165de6b4), rx_pos=ae0668(@f02b54f4)
  Version: 8 (min 3)

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

Marcelo Diop-Gonzalez (3):
  staging: vchiq_dump: Replace min with min_t
  staging: vchiq: Refactor indentation in vchiq_dump_* functions
  staging: vchiq: Have vchiq_dump_* functions return an error code

 .../interface/vchiq_arm/vchiq_2835_arm.c      |   5 +-
 .../interface/vchiq_arm/vchiq_arm.c           | 129 ++++++++++--------
 .../interface/vchiq_arm/vchiq_core.c          | 104 ++++++++++----
 .../interface/vchiq_arm/vchiq_core.h          |  12 +-
 4 files changed, 153 insertions(+), 97 deletions(-)


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