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