proc/vmcore: fix clearing user buffer by properly using clear_user()

In the Linux kernel, the following vulnerability has been resolved:

proc/vmcore: fix clearing user buffer by properly using clear_user()

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":

systemd[1]: Starting Kdump Vmcore Save Service...

kdump[420]: Kdump is using the default log level(3).

kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/

kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/

kdump[465]: saving vmcore-dmesg.txt complete

kdump[467]: saving vmcore

BUG: unable to handle page fault for address: 00007f2374e01000

#PF: supervisor write access in kernel mode

#PF: error_code(0x0003) - permissions violation

PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867

Oops: 0003 [#1] PREEMPT SMP NOPTI

CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014

RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86

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

RSP: 0018:ffffc9000073be08 EFLAGS: 00010212

RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000

RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008

RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50

R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000

R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8

FS: 00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0

Call Trace:

read_vmcore+0x236/0x2c0

proc_reg_read+0x55/0xa0

vfs_read+0x95/0x190

ksys_read+0x4f/0xc0

do_syscall_64+0x3b/0x90

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.