svn commit: r359892 - head/sys/kern
Mark Johnston
markj at FreeBSD.org
Mon Apr 13 19:20:39 UTC 2020
Author: markj
Date: Mon Apr 13 19:20:39 2020
New Revision: 359892
URL: https://svnweb.freebsd.org/changeset/base/359892
Log:
Relax restrictions on private mappings of POSIX shm objects.
When creating a private mapping of a POSIX shared memory object,
VM_PROT_WRITE should always be included in maxprot regardless of
permissions on the underlying FD. Otherwise it is possible to open a
shm object read-only, map it with MAP_PRIVATE and PROT_WRITE, and
violate the invariant in vm_map_insert() that (prot & maxprot) == prot.
Reported by: syzkaller
Reviewed by: kevans, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24398
Modified:
head/sys/kern/uipc_shm.c
Modified: head/sys/kern/uipc_shm.c
==============================================================================
--- head/sys/kern/uipc_shm.c Mon Apr 13 17:55:31 2020 (r359891)
+++ head/sys/kern/uipc_shm.c Mon Apr 13 19:20:39 2020 (r359892)
@@ -1136,23 +1136,28 @@ shm_mmap(struct file *fp, vm_map_t map, vm_offset_t *a
/*
* If FWRITE's set, we can allow VM_PROT_WRITE unless it's a shared
- * mapping with a write seal applied.
+ * mapping with a write seal applied. Private mappings are always
+ * writeable.
*/
- if ((fp->f_flag & FWRITE) != 0 && ((flags & MAP_SHARED) == 0 ||
- (shmfd->shm_seals & F_SEAL_WRITE) == 0))
+ if ((flags & MAP_SHARED) == 0) {
+ cap_maxprot |= VM_PROT_WRITE;
maxprot |= VM_PROT_WRITE;
+ writecnt = false;
+ } else {
+ if ((fp->f_flag & FWRITE) != 0 &&
+ (shmfd->shm_seals & F_SEAL_WRITE) == 0)
+ maxprot |= VM_PROT_WRITE;
+ writecnt = (prot & VM_PROT_WRITE) != 0;
+ if (writecnt && (shmfd->shm_seals & F_SEAL_WRITE) != 0) {
+ error = EPERM;
+ goto out;
+ }
- writecnt = (flags & MAP_SHARED) != 0 && (prot & VM_PROT_WRITE) != 0;
-
- if (writecnt && (shmfd->shm_seals & F_SEAL_WRITE) != 0) {
- error = EPERM;
- goto out;
- }
-
- /* Don't permit shared writable mappings on read-only descriptors. */
- if (writecnt && (maxprot & VM_PROT_WRITE) == 0) {
- error = EACCES;
- goto out;
+ /* Don't permit shared writable mappings on read-only descriptors. */
+ if (writecnt && (maxprot & VM_PROT_WRITE) == 0) {
+ error = EACCES;
+ goto out;
+ }
}
maxprot &= cap_maxprot;
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