Re: [PATCH 2/4] selftest/vm: verify remap destination address in mremap_test

[Sidhartha Kumar]

On 4/14/22 2:47 PM, Shuah Khan wrote:
> On 4/14/22 11:15 AM, Sidhartha Kumar wrote:
> > Because mremap does not have a NOREPLACE flag,
> > it can destroy existing mappings. This can
> > cause a segfault if regions such as text are
> > destroyed.
>
> Please explain the reason for segfault.
With the MREMAP_FIXED flag used by the test,
the text region, which fell in the range of the remap
region, got unmapped. This caused a segfault when
trying to fetch the next instruction after the mremap()
call.
> Add a blank line here. Makes it easier to read.
>
> Verify the requested mremap destination
> > address does not overlap any existing mappings
> > by using mmap's FIXED_NOREPLACE flag and checking
>
> Spell this out fully - MAP_FIXED_NOREPLACE
> > for the EEXIST error code. Keep incrementing the
> > destination address until a valid mapping is found
> > or max address is reached.
>
> Essentially mremap() doesn't check for overlaps and removes
> or overwrites existing mappings? The way you are fixing it
> is by verifying by calling mremap() with MAP_FIXED_NOREPLACE
> flag and check for EEXIST.
Yes, with the MREMAP_FIXED flag that the test uses, any previous
mapping in the address range of the remap region gets unmapped.
Yes, fixing this issue by calling mmap() with MAP_FIXED_NOREPLACE
flag and checking for EEXIST.

> What happens when max address is reached?
That is covered by the check if (addr > ULLONG_MAX - region size)
in the remap_region_valid() function.
> Same comment on # of chars per line in commit log. Also
>
> > Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
> > ---
> >   tools/testing/selftests/vm/mremap_test.c | 36 ++++++++++++++++++++++++
> >   1 file changed, 36 insertions(+)
> >
> > diff --git a/tools/testing/selftests/vm/mremap_test.c 
> > b/tools/testing/selftests/vm/mremap_test.c
> > index 58600fee4b81..98e9cff34aa7 100644
> > --- a/tools/testing/selftests/vm/mremap_test.c
> > +++ b/tools/testing/selftests/vm/mremap_test.c
> > @@ -10,6 +10,7 @@
> >   #include <string.h>
> >   #include <sys/mman.h>
> >   #include <time.h>
> > +#include <limits.h>
> >     #include "../kselftest.h"
> >   @@ -65,6 +66,34 @@ enum {
> >       .expect_failure = should_fail                \
> >   }
> >   +/*
> > + * Returns 0 if the requested remap region overlaps with an
> > + * existing mapping (e.g text, stack) else returns 1.
> > + */
> > +static int remap_region_valid(void *addr, unsigned long long size)
>
> This returns bool 0 (false) 1 (true)
>
> Please name the routine - is_remap_region_valid() and change it to
> return bool.
>
> > +{
> > +    void *remap_addr = NULL;
> > +    int ret = 1;
> > +
> > +    if ((unsigned long long) addr > ULLONG_MAX - size) {
> > +        ksft_print_msg("Can't find a valid region to remap to\n");
>
> Change it to "Couldn't" - also this message doesn't look right. We hav't
> looked for valid region yet and it just exceeds the limits?
Because this function is called in a loop in remap_region() and addr is 
being
incremented continuously, we could enter this function with addr high 
enough that
another increment would cause overflow.
> > +        exit(KSFT_SKIP);> +    }
> > +
> > +    /* Use MAP_FIXED_NOREPLACE flag to ensure region is not mapped */
> > +    remap_addr = mmap(addr, size, PROT_READ | PROT_WRITE,
> > +            MAP_FIXED_NOREPLACE | MAP_ANONYMOUS | MAP_SHARED,
> > +            -1, 0);
>
> Alignment should match open parenthesis here and in other places. 
> Makes it
> easier to read the code.
>
> > +    if (remap_addr == MAP_FAILED) {
> > +        if (errno == EEXIST)
> > +            ret = 0;
> > +    } else {
> > +        munmap(remap_addr, size);
> > +    }
> > +
> > +    return ret;
> > +}
> > +
> >   /* Returns mmap_min_addr sysctl */
> >   static unsigned long long get_mmap_min_addr(void)
> >   {
> > @@ -180,6 +209,13 @@ static long long remap_region(struct config c, 
> > unsigned int threshold_mb,
> >       if (!((unsigned long long) addr & c.dest_alignment))
> >           addr = (void *) ((unsigned long long) addr | 
> > c.dest_alignment);
> >   +    /* Don't destroy existing mappings unless expected to overlap */
> > +    while (!remap_region_valid(addr, c.region_size)) {
> > +        if (c.overlapping)
> > +            break;
> > +        addr += c.src_alignment;
> > +    }
> > +
> >       clock_gettime(CLOCK_MONOTONIC, &t_start);
> >       dest_addr = mremap(src_addr, c.region_size, c.region_size,
> >               MREMAP_MAYMOVE|MREMAP_FIXED, (char *) addr);
>
> thanks,
> -- Shuah