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vm: Fix anonymous memory clustering under ASLR

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By default, our ASLR implementation is supposed to cluster anonymous
memory allocations, unless the application's mmap(..., MAP_ANON, ...)
call included a non-zero address hint.  Unfortunately, clustering
never occurred because kern_mmap() always replaced the given address
hint when it was zero.  So, the ASLR implementation always believed
that a non-zero hint had been provided and randomized the mapping's
location in the address space.  To fix this problem, I'm pushing down
the point at which we convert a hint of zero to the minimum allocatable
address from kern_mmap() to vm_map_find_min().

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D40743
This commit is contained in:
Alan Cox 2023-06-23 12:00:32 -05:00
parent af296130ea
commit d8e6f4946c
3 changed files with 13 additions and 6 deletions
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10
sys/vm/vm_map.c
View file

@ -1981,14 +1981,14 @@ SYSCTL_INT(_vm, OID_AUTO, cluster_anon, CTLFLAG_RW,
"Cluster anonymous mappings: 0 = no, 1 = yes if no hint, 2 = always");
static bool
clustering_anon_allowed(vm_offset_t addr)
clustering_anon_allowed(vm_offset_t addr, int cow)
{
switch (cluster_anon) {
case 0:
return (false);
case 1:
return (addr == 0);
return (addr == 0 || (cow & MAP_NO_HINT) != 0);
case 2:
default:
return (true);
@ -2111,7 +2111,7 @@ vm_map_find(vm_map_t map, vm_object_t object, vm_ooffset_t offset,
} else
alignment = 0;
en_aslr = (map->flags & MAP_ASLR) != 0;
update_anon = cluster = clustering_anon_allowed(*addr) &&
update_anon = cluster = clustering_anon_allowed(*addr, cow) &&
(map->flags & MAP_IS_SUB_MAP) == 0 && max_addr == 0 &&
find_space != VMFS_NO_SPACE && object == NULL &&
(cow & (MAP_INHERIT_SHARE | MAP_STACK_GROWS_UP |
@ -2255,6 +2255,10 @@ vm_map_find_min(vm_map_t map, vm_object_t object, vm_ooffset_t offset,
int rv;
hint = *addr;
if (hint == 0)
cow |= MAP_NO_HINT;
if (hint < min_addr)
*addr = hint = min_addr;
for (;;) {
rv = vm_map_find(map, object, offset, addr, length, max_addr,
find_space, prot, max, cow);

1
sys/vm/vm_map.h
View file

@ -383,6 +383,7 @@ long vmspace_resident_count(struct vmspace *vmspace);
#define MAP_CREATE_STACK_GAP_DN 0x00020000
#define MAP_VN_EXEC 0x00040000
#define MAP_SPLIT_BOUNDARY_MASK 0x00180000
#define MAP_NO_HINT 0x00200000
#define MAP_SPLIT_BOUNDARY_SHIFT 19

8
sys/vm/vm_mmap.c
View file

@ -353,10 +353,12 @@ kern_mmap(struct thread *td, const struct mmap_req *mrp)
* the hint would fall in the potential heap space,
* place it after the end of the largest possible heap.
*
* There should really be a pmap call to determine a reasonable
* location.
* For anonymous mappings within the address space of the
* calling process, the absence of a hint is handled at a
* lower level in order to implement different clustering
* strategies for ASLR.
*/
if (addr == 0 ||
if (((flags & MAP_ANON) == 0 && addr == 0) ||
(addr >= round_page((vm_offset_t)vms->vm_taddr) &&
addr < round_page((vm_offset_t)vms->vm_daddr +
lim_max(td, RLIMIT_DATA))))