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.)
Cc: stable(a)vger.kernel.org
Fixes: af8ca1c14906 ("mm/mremap: optimize the start addresses in move_page_tables()")
Signed-off-by: Jann Horn <jannh(a)google.com>
---
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
```
---
mm/mremap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/mremap.c b/mm/mremap.c
index dda09e957a5d4c2546934b796e862e5e0213b311..dee98ff2bbd64439200dddac16c4bd054537c2ed 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -648,7 +648,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
* 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 */
---
base-commit: 2d5404caa8c7bb5c4e0435f94b28834ae5456623
change-id: 20241111-fix-mremap-32bit-wrap-747105730f20
--
Jann Horn <jannh(a)google.com>
From: David Wang <00107082(a)163.com>
[ Upstream commit 84b9749a3a704dcc824a88aa8267247c801d51e4 ]
seq_printf is costy, on a system with n CPUs, reading /proc/softirqs
would yield 10*n decimal values, and the extra cost parsing format string
grows linearly with number of cpus. Replace seq_printf with
seq_put_decimal_ull_width have significant performance improvement.
On an 8CPUs system, reading /proc/softirqs show ~40% performance
gain with this patch.
Signed-off-by: David Wang <00107082(a)163.com>
Signed-off-by: Linus Torvalds <torvalds(a)linux-foundation.org>
Signed-off-by: Sasha Levin <sashal(a)kernel.org>
---
fs/proc/softirqs.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/proc/softirqs.c b/fs/proc/softirqs.c
index 12901dcf57e2b..d8f4e7d54d002 100644
--- a/fs/proc/softirqs.c
+++ b/fs/proc/softirqs.c
@@ -19,7 +19,7 @@ static int show_softirqs(struct seq_file *p, void *v)
for (i = 0; i < NR_SOFTIRQS; i++) {
seq_printf(p, "%12s:", softirq_to_name[i]);
for_each_possible_cpu(j)
- seq_printf(p, " %10u", kstat_softirqs_cpu(i, j));
+ seq_put_decimal_ull_width(p, " ", kstat_softirqs_cpu(i, j), 10);
seq_putc(p, '\n');
}
return 0;
--
2.43.0