Re: Possible Bug in mincore or mmap

[Bruce Dubbs <bruce.dubbs@gmail.com>]

Nick Piggin wrote:
> Bruce Dubbs wrote:
>> When testing an installation with tests from the Linux Test Project, my
>> kernels fail one instance of the mincore01 tests:
>>
>> mincoremincore01    1  PASS  :  expected failure: errno = 22 (Invalid
>> argument)
>> mincore01    2  PASS  :  expected failure: errno = 14 (Bad address)
>> mincore01    3  FAIL  :  call succeeded unexpectedly
>> mincore01    4  PASS  :  expected failure: errno = 12 (Cannot allocate
>> memory)01    1  PASS  :  expected failure: errno = 22 (Invalid argument)
>> mincore01    2  PASS  :  expected failure: errno = 14 (Bad address)
>> mincore01    3  FAIL  :  call succeeded unexpectedly
>> mincore01    4  PASS  :  expected failure: errno = 12 (Cannot allocate
>> memory)
>>
>> I pared down the test to the attached program.  The result is supposed
>> to fail as it is asking for memory information 5 times what should be
>> allocated.
>>
>> Upon experimenting, I found the test works properly if a printf is
>> executed before the mmap call.  I have tested on locally built, but
>> unmodified, 2.4.25, 2.6.12.5, and a 2.6.20.3 kernels and get the same
>> behavior.  The tests fail on IA32 architecture, but not 64-bit kernels.
>>  The test always works properly on FC6 and RHEL3.
>>
>> I've checked the archives for this issue and could not find anything
>> appropriate.
>>
>> Could this be a potential security issue as memory that is not supposed
>> to be accessible seems to be available to the user?  Is it expected
>> behavior?
> 
> It shouldn't be a security problem if mincore doesn't actually
> return the data.

Thanks for the response.  It may be interesting to note that adding:

buf = (char*)global_pointer + 2 * global_len;
i = *buf;

after the mincore call does not fail. Changing the 2nd line above to
*buf = 1; gives a segmentation fault as you would expect.

As a minimum, it appears the mmap function is allowing read access
beyond its allocated address space in some circumstances.

Upon thinking about it, it may be that the test is invalid.  I don't
believe there is anything tying the mincore query to the memory region
allocated by mmap.  If memory mapping occurs beyond the mmap requested
memory size to anticipate another memory request, mincore wouldn't fail.

Does this make any sense?




>> ------------------------------------------------------------------------
>>
>> #include <sys/mman.h>
>> #include <stdlib.h>
>> #include <string.h>
>> #include <stdio.h>
>>
>> static int   PAGESIZE;
>> static char  file_name[]    = "fooXXXXXX";
>> static void* global_pointer = NULL;
>> static int   global_len     = 0;
>> static int   file_desc      = 0;
>>
>> int main(int argc, char **argv)
>> {
>>     int             i;
>>     int             result;
>>     char*           buf;
>>     unsigned char   vect[20] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};
>>    
>>     PAGESIZE = getpagesize();
>>         /* global_pointer will point to a mmapped area of global_len
>> bytes */
>>     global_len = PAGESIZE*2;
>>         buf = (char*)malloc(global_len);
>>     memset(buf, 42, global_len);  // Asterisks         /* create a
>> temporary file */
>>     file_desc = mkstemp(file_name);
>>         /* fill the temporary file with two pages of data */
>>     write(file_desc, buf, global_len);
>>     free(buf);
>>         // Will work properly as long as print is before mmap function.
>>     if ( argc > 1 ) printf("argc=%d\n", argc);
>>
>>     /* map the file in memory */
>>     global_pointer = mmap( NULL, global_len,
>>             PROT_READ|PROT_WRITE|PROT_EXEC, MAP_SHARED, file_desc, 0);
>>
>>     // Result should be -1 as the request is 5 times actual mapping
>>     result = mincore(global_pointer, (size_t)(global_len*5), vect);
>>
>>     // Print some data
>>     printf("PAGESIZE=%d\n", PAGESIZE);
>>     printf("global_len=%d\n", global_len);
>>     printf("global_pointer=0x%x\n", (unsigned int)global_pointer);
>>     printf("alloc=%d\n", (global_len+PAGESIZE-1) / PAGESIZE );
>>     printf("Result=%d\n", result);
>>     printf("vect: ");
>>
>>     for ( i=0; i<20; i++) printf("%02x ", vect[i]);
>>     printf("\n");
>>         // Clean up
>>     munmap(global_pointer, (size_t)global_len);
>>     close(file_desc);
>>     unlink(file_name);
>> }
> 
>