On 2019-01-03 16:13, Michal Hocko wrote:
On Thu 03-01-19 15:59:52, Roman Penyaev wrote:
area->size can include adjacent guard page but get_vm_area_size()
returns actual size of the area.
This fixes possible kernel crash when userspace tries to map area
on 1 page bigger: size check passes but the following
vmalloc_to_page()
returns NULL on last guard (non-existing) page.
Can this actually happen? I am not really familiar with all the callers
of this API but VM_NO_GUARD is not really used wildly in the kernel.
Exactly, by default (VM_NO_GUARD is not set) each area has guard page,
thus the area->size will be bigger. The bug is not reproduced if
VM_NO_GUARD is set.
All I can see is kasan na arm64 which doesn't really seem to use it
for vmalloc.
So is the problem real or this is a mere cleanup?
This is the real problem, try this hunk for any file descriptor which
provides
mapping, or say modify epoll as example:
--------------------------------
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
+static int ep_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ void *mem;
+
+ mem = vmalloc_user(4096);
+ BUG_ON(!mem);
+ /* Do not care about mem leak */
+
+ return remap_vmalloc_range(vma, mem, 0);
+}
+
/* File callbacks that implement the eventpoll file behaviour */
static const struct file_operations eventpoll_fops = {
#ifdef CONFIG_PROC_FS
.show_fdinfo = ep_show_fdinfo,
#endif
+ .mmap = ep_mmap,
.release = ep_eventpoll_release,
--------------------------------
and the following code from userspace, which maps 2 pages,
instead of 1:
--------------------------------
epfd = epoll_create1(0);
assert(epfd >= 0);
p = mmap(NULL, 2<<12, PROT_WRITE|PROT_READ, MAP_PRIVATE, epfd, 0);
assert(p != MAP_FAILED);
--------------------------------
You immediately get the following oops:
[ 38.894571] BUG: unable to handle kernel NULL pointer dereference at
0000000000000008
[ 38.899048] #PF error: [normal kernel read fault]
[ 38.901487] PGD 0 P4D 0
[ 38.902801] Oops: 0000 [#1] PREEMPT SMP PTI
[ 38.904984] CPU: 2 PID: 399 Comm: mmap-epoll Not tainted 4.20.0-1
#238
[ 38.914064] RIP: 0010:vm_insert_page+0x3b/0x1d0
[ 38.941181] Call Trace:
[ 38.941656] remap_vmalloc_range_partial+0x8d/0xd0
[ 38.942417] mmap_region+0x3c7/0x630
[ 38.942982] do_mmap+0x38d/0x560
[ 38.943479] vm_mmap_pgoff+0x9a/0xf0
[ 38.944028] ksys_mmap_pgoff+0x18e/0x220
[ 38.944554] do_syscall_64+0x48/0xf0
[ 38.945076] entry_SYSCALL_64_after_hwframe+0x44/0xa9
--
Roman
