CVE-2021-47566

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> proc/vmcore: fix clearing user buffer by properly using clear_user()<br /> <br /> To clear a user buffer we cannot simply use memset, we have to use<br /> clear_user(). With a virtio-mem device that registers a vmcore_cb and<br /> has some logically unplugged memory inside an added Linux memory block,<br /> I can easily trigger a BUG by copying the vmcore via "cp":<br /> <br /> systemd[1]: Starting Kdump Vmcore Save Service...<br /> kdump[420]: Kdump is using the default log level(3).<br /> kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/<br /> kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/<br /> kdump[465]: saving vmcore-dmesg.txt complete<br /> kdump[467]: saving vmcore<br /> BUG: unable to handle page fault for address: 00007f2374e01000<br /> #PF: supervisor write access in kernel mode<br /> #PF: error_code(0x0003) - permissions violation<br /> PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867<br /> Oops: 0003 [#1] PREEMPT SMP NOPTI<br /> CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6<br /> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014<br /> RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86<br /> 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 c7 45 00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81<br /> RSP: 0018:ffffc9000073be08 EFLAGS: 00010212<br /> RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000<br /> RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008<br /> RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50<br /> R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000<br /> R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8<br /> FS: 00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000<br /> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033<br /> CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0<br /> Call Trace:<br /> read_vmcore+0x236/0x2c0<br /> proc_reg_read+0x55/0xa0<br /> vfs_read+0x95/0x190<br /> ksys_read+0x4f/0xc0<br /> do_syscall_64+0x3b/0x90<br /> entry_SYSCALL_64_after_hwframe+0x44/0xae<br /> <br /> Some x86-64 CPUs have a CPU feature called "Supervisor Mode Access<br /> Prevention (SMAP)", which is used to detect wrong access from the kernel<br /> to user buffers like this: SMAP triggers a permissions violation on<br /> wrong access. In the x86-64 variant of clear_user(), SMAP is properly<br /> handled via clac()+stac().<br /> <br /> To fix, properly use clear_user() when we&amp;#39;re dealing with a user buffer.