6.11-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jann Horn jannh@google.com
commit a4a282daf1a190f03790bf163458ea3c8d28d217 upstream.
On 32-bit platforms, it is possible for the expression `len + old_addr < old_end` to be false-positive if `len + old_addr` wraps around. `old_addr` is the cursor in the old range up to which page table entries have been moved; so if the operation succeeded, `old_addr` is the *end* of the old region, and adding `len` to it can wrap.
The overflow causes mremap() to mistakenly believe that PTEs have been copied; the consequence is that mremap() bails out, but doesn't move the PTEs back before the new VMA is unmapped, causing anonymous pages in the region to be lost. So basically if userspace tries to mremap() a private-anon region and hits this bug, mremap() will return an error and the private-anon region's contents appear to have been zeroed.
The idea of this check is that `old_end - len` is the original start address, and writing the check that way also makes it easier to read; so fix the check by rearranging the comparison accordingly.
(An alternate fix would be to refactor this function by introducing an "orig_old_start" variable or such.)
Tested in a VM with a 32-bit X86 kernel; without the patch:
``` user@horn:~/big_mremap$ cat test.c #define _GNU_SOURCE #include <stdlib.h> #include <stdio.h> #include <err.h> #include <sys/mman.h>
#define ADDR1 ((void*)0x60000000) #define ADDR2 ((void*)0x10000000) #define SIZE 0x50000000uL
int main(void) { unsigned char *p1 = mmap(ADDR1, SIZE, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED_NOREPLACE, -1, 0); if (p1 == MAP_FAILED) err(1, "mmap 1"); unsigned char *p2 = mmap(ADDR2, SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED_NOREPLACE, -1, 0); if (p2 == MAP_FAILED) err(1, "mmap 2"); *p1 = 0x41; printf("first char is 0x%02hhx\n", *p1); unsigned char *p3 = mremap(p1, SIZE, SIZE, MREMAP_MAYMOVE|MREMAP_FIXED, p2); if (p3 == MAP_FAILED) { printf("mremap() failed; first char is 0x%02hhx\n", *p1); } else { printf("mremap() succeeded; first char is 0x%02hhx\n", *p3); } } user@horn:~/big_mremap$ gcc -static -o test test.c user@horn:~/big_mremap$ setarch -R ./test first char is 0x41 mremap() failed; first char is 0x00 ```
With the patch:
``` user@horn:~/big_mremap$ setarch -R ./test first char is 0x41 mremap() succeeded; first char is 0x41 ```
Link: https://lkml.kernel.org/r/20241111-fix-mremap-32bit-wrap-v1-1-61d6be73b722@g... Fixes: af8ca1c14906 ("mm/mremap: optimize the start addresses in move_page_tables()") Signed-off-by: Jann Horn jannh@google.com Acked-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com Acked-by: Qi Zheng zhengqi.arch@bytedance.com Reviewed-by: Liam R. Howlett Liam.Howlett@Oracle.com Cc: Joel Fernandes (Google) joel@joelfernandes.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/mremap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/mremap.c +++ b/mm/mremap.c @@ -648,7 +648,7 @@ again: * Prevent negative return values when {old,new}_addr was realigned * but we broke out of the above loop for the first PMD itself. */ - if (len + old_addr < old_end) + if (old_addr < old_end - len) return 0;
return len + old_addr - old_end; /* how much done */