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userfaultfd: selftests: fix SIGSEGV if huge mmap fails 

The error handling in hugetlb_allocate_area() was incorrect for the
hugetlb_shared test case.

Previously the behavior was:

- mmap a hugetlb area
  - If this fails, set the pointer to NULL, and carry on
- mmap an alias of the same hugetlb fd
  - If this fails, munmap the original area

If the original mmap failed, it's likely the second one did too.  If
both failed, we'd blindly try to munmap a NULL pointer, causing a
SIGSEGV.  Instead, "goto fail" so we return before trying to mmap the
alias.

This issue can be hit "in real life" by forgetting to set
/proc/sys/vm/nr_hugepages (leaving it at 0), and then trying to run the
hugetlb_shared test.

Another small improvement is, when the original mmap fails, don't just
print "it failed": perror(), so we can see *why*.  :)

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Joe Perches <joe@perches.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Alan Gilbert <dgilbert@redhat.com>
Link: https://lkml.kernel.org/r/20201204203443.2714693-1-axelrasmussen@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Axel Rasmussen 2020-12-05 22:15:05 -08:00 committed by Linus Torvalds
parent d8cbe8bfa7
commit 573a259336
1 changed files with 16 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
tools/testing/selftests/vm/userfaultfd.c
View File

@ -206,19 +206,19 @@ static int hugetlb_release_pages(char *rel_area)
return ret; return ret;
} }
static void hugetlb_allocate_area(void **alloc_area) static void hugetlb_allocate_area(void **alloc_area)
{ {
void *area_alias = NULL; void *area_alias = NULL;
char **alloc_area_alias; char **alloc_area_alias;
*alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE, *alloc_area = mmap(NULL, nr_pages * page_size, PROT_READ | PROT_WRITE,
(map_shared ? MAP_SHARED : MAP_PRIVATE) | (map_shared ? MAP_SHARED : MAP_PRIVATE) |
MAP_HUGETLB, MAP_HUGETLB,
huge_fd, *alloc_area == area_src ? 0 : huge_fd, *alloc_area == area_src ? 0 :
nr_pages * page_size); nr_pages * page_size);
if (*alloc_area == MAP_FAILED) { if (*alloc_area == MAP_FAILED) {
fprintf(stderr, "mmap of hugetlbfs file failed\n"); perror("mmap of hugetlbfs file failed");
*alloc_area = NULL; goto fail;
} }
if (map_shared) { if (map_shared) {
@ -227,14 +227,11 @@ static void hugetlb_allocate_area(void **alloc_area)
huge_fd, *alloc_area == area_src ? 0 : huge_fd, *alloc_area == area_src ? 0 :
nr_pages * page_size); nr_pages * page_size);
if (area_alias == MAP_FAILED) { if (area_alias == MAP_FAILED) {
if (munmap(*alloc_area, nr_pages * page_size) < 0) { perror("mmap of hugetlb file alias failed");
perror("hugetlb munmap"); goto fail_munmap;
exit(1);
}
*alloc_area = NULL;
return;
} }
} }
if (*alloc_area == area_src) { if (*alloc_area == area_src) {
huge_fd_off0 = *alloc_area; huge_fd_off0 = *alloc_area;
alloc_area_alias = &area_src_alias; alloc_area_alias = &area_src_alias;
@ -243,6 +240,16 @@ static void hugetlb_allocate_area(void **alloc_area)
} }
if (area_alias) if (area_alias)
*alloc_area_alias = area_alias; *alloc_area_alias = area_alias;
return;
fail_munmap:
if (munmap(*alloc_area, nr_pages * page_size) < 0) {
perror("hugetlb munmap");
exit(1);
}
fail:
*alloc_area = NULL;
} }
static void hugetlb_alias_mapping(__u64 *start, size_t len, unsigned long offset) static void hugetlb_alias_mapping(__u64 *start, size_t len, unsigned long offset)