Title

A vulnerability in Linux kernel mainline v5.18-rc1 through v5.19-rc6 does not clear statically allocated variables in the block starting symbol (.bss) due to a failed early_xen_iret_patch leading to an asm_exc_page_fault, or arbitrary code execution

CVE ID

CVE-2022-36123

CVSS Score

N/A

Internal IDs

SICK-2022-128

Vendor

Kernel.org

Product

Linux Kernel

Product Versions

Kernel v5.18-rc1 through v5.19-rc6

Vulnerability Details

A vulnerability in Linux kernel mainline v5.18-rc1 through v5.19-rc6 may not clear the block starting symbol (.bss) where statically allocated variables in the .bss, affecting XenPV guests, leading to an asm_exc_page_fault, or arbitrary code execution. An unprivileged local attacker on the host, or guest, may potentially use this flaw to cause a NULL Pointer Dereference, kernel oops or denial of service as this allows virtualized devices connected to the Xen IOMMU via xen_set_restricted_virtio_memory_access to potentially access restricted memory. In addition, if kexec is used, the 2nd kernel .bss may contain uninitialized resources and may not be clear.

Vendor Response

Fixed in kernel mainline. clear_bss() now clears the .brk at early boot and xen_set_restricted_virtio_memory_access was added via CONFIG_XEN_VIRTIO kernel config.

Fix: 36e2f161fb01795722f2ff1a24d95f08100333dd

Upstream: 38fa5479b41376dc9d7f57e71c83514285a25ca0

Fixed in 5.18.13 stable https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.18.13

Fixed in 5.15.56 longterm 5.15.x https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.15.56

Fixed in 5.10.132 longterm 5.10.x https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.10.132

Fixed in 5.4.207 longterm 5.4.x https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.4.207

Fixed in 4.19.253 longterm 4.19.x https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.19.253

Fixed in 4.14.289 longterm 4.14.x https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.289

Fixed in 4.9.324 longterm 4.9.324 https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.9.324

Proof of Concept

A note was left when v5.18-rc1 was released describing the use of xen_start_info arch/x86/xen/mmu_pv.c: "The xen_start_info has been taken care of already in xen_setup_kernel_pagetable"

https://github.com/torvalds/linux/blob/babf0bb978e3c9fce6c4eba6b744c8754fd43d8e/arch/x86/xen/mmu_pv.c#L1151

After early_xen_iret_patch was added, it was later urgently removed & cleaned up in x86_urgent_for_v5.19_rc6:

 Merge tag 'x86_urgent_for_v5.19_rc6' of git://git.kernel.org/pub/scm/…

…linux/kernel/git/tip/tip

Pull x86 fixes from Borislav Petkov:

 - Prepare for and clear .brk early in order to address XenPV guests
   failures where the hypervisor verifies page tables and uninitialized
   data in that range leads to bogus failures in those checks

 - Add any potential setup_data entries supplied at boot to the identity
   pagetable mappings to prevent kexec kernel boot failures. Usually,
   this is not a problem for the normal kernel as those mappings are
   part of the initially mapped 2M pages but if kexec gets to allocate
   the second kernel somewhere else, those setup_data entries need to be
   mapped there too.

 - Fix objtool not to discard text references from the __tracepoints
   section so that ENDBR validation still works

 - Correct the setup_data types limit as it is user-visible, before 5.19
   releases

* tag 'x86_urgent_for_v5.19_rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/boot: Fix the setup data types max limit
  x86/ibt, objtool: Don't discard text references from tracepoint section
  x86/compressed/64: Add identity mappings for setup_data entries
  x86: Fix .brk attribute in linker script
  x86: Clear .brk area at early boot
  x86/xen: Use clear_bss() for Xen PV guests

If clear_bss() is not added, and a user runs kexec, the "normal kernel... mappings are part of the initially mapped 2M pages but kexec gets to allocate the second kernel somewhere else, those setup_data entries need to be mapped there too."

* tag 'x86_urgent_for_v5.19_rc6' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/boot: Fix the setup data types max limit
  x86/ibt, objtool: Don't discard text references from tracepoint section
  x86/compressed/64: Add identity mappings for setup_data entries
  x86: Fix .brk attribute in linker script
  x86: Clear .brk area at early boot
  x86/xen: Use clear_bss() for Xen PV guests

Virtualized devices connected to the Xen IOMMU can potentially access restricted memory.

[12002.517482] BUG: kernel NULL pointer dereference, address: 0000000000000344
[12002.517487] #PF: supervisor write access in kernel mode
[12002.517489] #PF: error_code(0x0002) - not-present page
[12002.517490] PGD 0 P4D 0
[12002.517493] Oops: 0002 [#1] PREEMPT SMP NOPTI
[12002.517499] RIP: 0010:copy_fpstate_to_sigframe+0xad/0x330
[12002.517505] Code: 1f 44 00 00 48 8d bd 00 02 00 00 be 40 00 00 00 e8 b8 eb 58 00 48 85 c0 75 bc 65 48 8b 1c 25 c0 0b 02 00 65 81 05 9f 73 1e 5d <00> 02 00 00 48 8b 03 f6 c4 40 0f 85 ab 00 00 00 83 83 f8 1a 00 00
[12002.517506] RSP: 0000:ffffb298c2fb3df0 EFLAGS: 00050212
[12002.517508] RAX: 000000005d1e747a RBX: ffffb298c2fb3f58 RCX: 0000000000000008
[12002.517510] RDX: 0000000000000344 RSI: 0000000000000040 RDI: 00007fdb33e8ee80
[12002.517511] RBP: 00007fdb33e8ec80 R08: ffff9510262ef640 R09: 0000000000000000
[12002.517512] R10: 000000000000000b R11: 000000000000000a R12: ffff950c8fca5d40
[12002.517513] R13: ffff950c8fca4080 R14: ffff950c8fca4080 R15: 00007fdb33e8ec80
[12002.517515] FS:  00007fdb32000340(0000) GS:ffff95128f600000(0000) knlGS:0000000000000000
[12002.517516] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[12002.517517] CR2: 0000000000000344 CR3: 0000000204082000 CR4: 0000000000350ef0
[12002.517520] Call Trace:
[12002.517521]  <TASK>
[12002.517522]  ? copy_fpstate_to_sigframe+0x98/0x330
[12002.517525]  ? get_signal+0x7f2/0x990
[12002.517528]  ? arch_do_signal_or_restart+0x64d/0x760
[12002.517531]  ? early_xen_iret_patch+0x5/0xc
[12002.517535]  ? exit_to_user_mode_prepare+0xd3/0x140
[12002.517538]  ? asm_exc_page_fault+0xc/0x30
[12002.517540]  ? irqentry_exit_to_user_mode+0x9/0x20
[12002.517542]  ? asm_exc_page_fault+0x22/0x30
[12002.517545]  ? early_xen_iret_patch+0x5/0xc
[12002.517547]  </TASK>
...
[12002.517625] CR2: 0000000000000344
[12002.517627] ---[ end trace 0000000000000000 ]---
[12002.517629] RIP: 0010:copy_fpstate_to_sigframe+0xad/0x330
[12002.517631] Code: 1f 44 00 00 48 8d bd 00 02 00 00 be 40 00 00 00 e8 b8 eb 58 00 48 85 c0 75 bc 65 48 8b 1c 25 c0 0b 02 00 65 81 05 9f 73 1e 5d <00> 02 00 00 48 8b 03 f6 c4 40 0f 85 ab 00 00 00 83 83 f8 1a 00 00
[12002.517632] RSP: 0000:ffffb298c2fb3df0 EFLAGS: 00050212
[12002.517634] RAX: 000000005d1e747a RBX: ffffb298c2fb3f58 RCX: 0000000000000008
[12002.517635] RDX: 0000000000000344 RSI: 0000000000000040 RDI: 00007fdb33e8ee80
[12002.517636] RBP: 00007fdb33e8ec80 R08: ffff9510262ef640 R09: 0000000000000000
[12002.517637] R10: 000000000000000b R11: 000000000000000a R12: ffff950c8fca5d40
[12002.517638] R13: ffff950c8fca4080 R14: ffff950c8fca4080 R15: 00007fdb33e8ec80
[12002.517639] FS:  00007fdb32000340(0000) GS:ffff95128f600000(0000) knlGS:0000000000000000
[12002.517641] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[12002.517642] CR2: 0000000000000344 CR3: 0000000204082000 CR4: 0000000000350ef0

Kernel Changelog:

commit a3c7c1a726a4c6b63b85e8c183f207543fd75e1b
Author: Juergen Gross <jgross@suse.com>
Date:   Thu Jun 30 09:14:40 2022 +0200

    x86: Clear .brk area at early boot
    
    [ Upstream commit 38fa5479b41376dc9d7f57e71c83514285a25ca0 ]
    
    The .brk section has the same properties as .bss: it is an alloc-only
    section and should be cleared before being used.
    
    Not doing so is especially a problem for Xen PV guests, as the
    hypervisor will validate page tables (check for writable page tables
    and hypervisor private bits) before accepting them to be used.
    
    Make sure .brk is initially zero by letting clear_bss() clear the brk
    area, too.
    
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Link: https://lore.kernel.org/r/20220630071441.28576-3-jgross@suse.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>

And early_xen_iret_patch was added https://github.com/torvalds/linux/commit/8b87d8cec1b31ea710568ae49ba5f5146318da0d

Then later urgently removed:

x86/xen: Use clear_bss() for Xen PV guests

https://github.com/torvalds/linux/commit/96e8fc5818686d4a1591bb6907e7fdb64ef29884

And cleaned up in x86_urgent_for_v5.19_rc6:

https://github.com/torvalds/linux/commit/74a0032b8524ee2bd4443128c0bf9775928680b0

CONFIG_XEN_VIRTIO was added:

https://github.com/torvalds/linux/commit/fa1f57421e0b1c57843902c89728f823abc32f02

Upstream: 38fa5479b41376dc9d7f57e71c83514285a25ca0

Fix: 36e2f161fb01795722f2ff1a24d95f08100333dd

Disclosure Timeline

2022-07-10 - Borislav Petkov & Juergen Gross fixes the vulnerability in mainline 5.19.rc6
2022-07-14 - Researcher encounters & reports vulnerability vulnerability on stable 5.18.11.