To clear a user buffer we cannot simply use memset, we have to use
clear_user(). With a virtio-mem device that registers a vmcore_cb and has
some logically unplugged memory inside an added Linux memory block, I can
easily trigger a BUG by copying the vmcore via "cp":
[ 11.327580] systemd[1]: Starting Kdump Vmcore Save Service...
[ 11.339697] kdump[420]: Kdump is using the default log level(3).
[ 11.370964] kdump[453]: saving to
/sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
[ 11.373997] kdump[458]: saving vmcore-dmesg.txt to
/sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
[ 11.385357] kdump[465]: saving vmcore-dmesg.txt complete
[ 11.386722] kdump[467]: saving vmcore
[ 16.531275] BUG: unable to handle page fault for address: 7f2374e01000
[ 16.531705] #PF: supervisor write access in kernel mode
[ 16.532037] #PF: error_code(0x0003) - permissions violation
[ 16.532396] PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE
80007048f867
[ 16.532872] Oops: 0003 [#1] PREEMPT SMP NOPTI
[ 16.533154] CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6
[ 16.533513] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014
[ 16.534198] RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86
[ 16.534552] Code: ff ff ff e8 05 ff fe ff e9 b9 e9 7f ff 48 89 de 48 c7 c7
38 3b 60 82 e8 f1 fe fe ff 83 fd 08 72 3c 49 8d 7d 08 4c 89 e9 89 e8 <49> c7 45
00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81
[ 16.535670] RSP: 0018:c973be08 EFLAGS: 00010212
[ 16.535998] RAX: 1000 RBX: 002fd000 RCX: 7f2374e01000
[ 16.536441] RDX: 0001 RSI: dfff RDI: 7f2374e01008
[ 16.536878] RBP: 1000 R08: R09: c973bc50
[ 16.537315] R10: c973bc48 R11: 829461a8 R12: f000
[ 16.537755] R13: 7f2374e01000 R14: R15: 88807bd421e8
[ 16.538200] FS: 7f2374e12140() GS:88807f00()
knlGS:
[ 16.538696] CS: 0010 DS: ES: CR0: 80050033
[ 16.539055] CR2: 7f2374e01000 CR3: 7a4aa000 CR4: 00350eb0
[ 16.539510] Call Trace:
[ 16.539679]
[ 16.539828] read_vmcore+0x236/0x2c0
[ 16.540063] ? enqueue_hrtimer+0x2f/0x80
[ 16.540323] ? inode_security+0x22/0x60
[ 16.540572] proc_reg_read+0x55/0xa0
[ 16.540807] vfs_read+0x95/0x190
[ 16.541022] ksys_read+0x4f/0xc0
[ 16.541238] do_syscall_64+0x3b/0x90
[ 16.541475] entry_SYSCALL_64_after_hwframe+0x44/0xae
Some x86-64 CPUs have a CPU feature called "Supervisor Mode Access
Prevention (SMAP)", which is used to detect wrong access from the kernel to
user buffers like this: SMAP triggers a permissions violation on wrong
access. In the x86-64 variant of clear_user(), SMAP is properly
handled via clac()+stac().
To fix, properly use clear_user() when we're dealing with a user buffer.
Fixes: 997c136f518c ("fs/proc/vmcore.c: add hook to read_from_oldmem() to check
for non-ram pages")
Acked-by: Baoquan He
Cc: Dave Young
Cc: Baoquan He
Cc: Vivek Goyal
Cc: Andrew Morton
Cc: Philipp Rudo
Cc: kexec@lists.infradead.org
Cc: linux...@kvack.org
Cc: linux-fsde...@vger.kernel.org
Signed-off-by: David Hildenbrand
---
v1 -> v2:
- Extend patch description
---
fs/proc/vmcore.c | 20
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 30a3b66f475a..509f85148fee 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -154,9 +154,13 @@ ssize_t read_from_oldmem(char *buf, size_t count,
nr_bytes = count;
/* If pfn is not ram, return zeros for sparse dump files */
- if (!pfn_is_ram(pfn))
- memset(buf, 0, nr_bytes);
- else {
+ if (!pfn_is_ram(pfn)) {
+ tmp = 0;
+ if (!userbuf)
+ memset(buf, 0, nr_bytes);
+ else if (clear_user(buf, nr_bytes))
+ tmp = -EFAULT;
+ } else {
if (encrypted)
tmp = copy_oldmem_page_encrypted(pfn, buf,
nr_bytes,
@@ -165,12 +169,12 @@ ssize_t read_from_oldmem(char *buf, size_t count,
else
tmp = copy_oldmem_page(pfn, buf, nr_bytes,
offset, userbuf);
-
- if (tmp < 0) {
- up_read(_cb_rwsem);
- return tmp;
- }
}
+ if (tmp < 0) {
+ up_read(_cb_rwsem);
+ return tmp;
+ }
+
*ppos += nr_bytes;
count -=
