From: David Hildenbrand david@redhat.com Subject: 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":

[ 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: 00007f2374e01000 [ 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 800000007048f867 [ 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:ffffc9000073be08 EFLAGS: 00010212 [ 16.535998] RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000 [ 16.536441] RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008 [ 16.536878] RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50 [ 16.537315] R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000 [ 16.537755] R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8 [ 16.538200] FS: 00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000 [ 16.538696] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 16.539055] CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0 [ 16.539510] Call Trace: [ 16.539679] <TASK> [ 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.

Link: https://lkml.kernel.org/r/20211112092750.6921-1-david@redhat.com Fixes: 997c136f518c ("fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages") Signed-off-by: David Hildenbrand david@redhat.com Acked-by: Baoquan He bhe@redhat.com Cc: Dave Young dyoung@redhat.com Cc: Baoquan He bhe@redhat.com Cc: Vivek Goyal vgoyal@redhat.com Cc: Philipp Rudo prudo@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/proc/vmcore.c | 20 ++++++++++++-------- 1 file changed, 12 insertions(+), 8 deletions(-)

--- a/fs/proc/vmcore.c~proc-vmcore-fix-clearing-user-buffer-by-properly-using-clear_user +++ a/fs/proc/vmcore.c @@ -154,9 +154,13 @@ ssize_t read_from_oldmem(char *buf, size 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 else tmp = copy_oldmem_page(pfn, buf, nr_bytes, offset, userbuf); - - if (tmp < 0) { - up_read(&vmcore_cb_rwsem); - return tmp; - } } + if (tmp < 0) { + up_read(&vmcore_cb_rwsem); + return tmp; + } + *ppos += nr_bytes; count -= nr_bytes; buf += nr_bytes; _