On 11/11/21 at 08:18pm, David Hildenbrand wrote:
> To clear a user buffer we cannot simply use memset, we have to use
> clear_user(). Using a kernel config based on rawhide Fedora and a
> virtio-mem device that registers a vmcore_cb, I can easily trigger:
>
> [ 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
>
> To fix, properly use clear_user() when required.
Looks a great fix to me, thanks for fixing this.
Check the code, clear_user invokes access_ok to do check, then call
memset(). It's unclear to me how the bug is triggered, could you
please tell more so that I can learn?
>
> Fixes: 997c136f518c ("fs/proc/vmcore.c: add hook to read_from_oldmem() to check for non-ram pages")
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Philipp Rudo <prudo@xxxxxxxxxx>
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
> 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(&vmcore_cb_rwsem);
> - return tmp;
> - }
> }
> + if (tmp < 0) {
> + up_read(&vmcore_cb_rwsem);
> + return tmp;
> + }
> +
> *ppos += nr_bytes;
> count -= nr_bytes;
> buf += nr_bytes;
>
> base-commit: debe436e77c72fcee804fb867f275e6d31aa999c
> --
> 2.31.1
>
