Subject: CVE-2024-42240: x86/bhi: Avoid warning in #DB handler due to BHI mitigation

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

x86/bhi: Avoid warning in #DB handler due to BHI mitigation

When BHI mitigation is enabled, if SYSENTER is invoked with the TF flag set
then entry_SYSENTER_compat() uses CLEAR_BRANCH_HISTORY and calls the
clear_bhb_loop() before the TF flag is cleared. This causes the #DB handler
(exc_debug_kernel()) to issue a warning because single-step is used outside the
entry_SYSENTER_compat() function.

To address this issue, entry_SYSENTER_compat() should use CLEAR_BRANCH_HISTORY
after making sure the TF flag is cleared.

The problem can be reproduced with the following sequence:

  $ cat sysenter_step.c
  int main()
  { asm("pushf; pop %ax; bts $8,%ax; push %ax; popf; sysenter"); }

  $ gcc -o sysenter_step sysenter_step.c

  $ ./sysenter_step
  Segmentation fault (core dumped)

The program is expected to crash, and the #DB handler will issue a warning.

Kernel log:

  WARNING: CPU: 27 PID: 7000 at arch/x86/kernel/traps.c:1009 exc_debug_kernel+0xd2/0x160
  ...
  RIP: 0010:exc_debug_kernel+0xd2/0x160
  ...
  Call Trace:
  <#DB>
   ? show_regs+0x68/0x80
   ? __warn+0x8c/0x140
   ? exc_debug_kernel+0xd2/0x160
   ? report_bug+0x175/0x1a0
   ? handle_bug+0x44/0x90
   ? exc_invalid_op+0x1c/0x70
   ? asm_exc_invalid_op+0x1f/0x30
   ? exc_debug_kernel+0xd2/0x160
   exc_debug+0x43/0x50
   asm_exc_debug+0x1e/0x40
  RIP: 0010:clear_bhb_loop+0x0/0xb0
  ...
  </#DB>
  <TASK>
   ? entry_SYSENTER_compat_after_hwframe+0x6e/0x8d
  </TASK>

  [ bp: Massage commit message. ]

The Linux kernel CVE team has assigned CVE-2024-42240 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.15.154 with commit bd53ec80f218 and fixed in 5.15.163 with commit db56615e96c4
	Issue introduced in 6.1.85 with commit 07dbb10f153f and fixed in 6.1.100 with commit a765679defe1
	Issue introduced in 6.6.26 with commit eb36b0dce213 and fixed in 6.6.41 with commit dae3543db8f0
	Issue introduced in 6.9 with commit 7390db8aea0d and fixed in 6.9.10 with commit 08518d48e5b7
	Issue introduced in 6.9 with commit 7390db8aea0d and fixed in 6.10 with commit ac8b270b61d4
	Issue introduced in 6.8.5 with commit 8f51637712e4

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-42240
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/entry/entry_64_compat.S

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/db56615e96c439e13783d7715330e824b4fd4b84
	https://git.kernel.org/stable/c/a765679defe1dc1b8fa01928a6ad6361e72a1364
	https://git.kernel.org/stable/c/dae3543db8f0cf8ac1a198c3bb4b6e3c24d576cf
	https://git.kernel.org/stable/c/08518d48e5b744620524f0acd7c26c19bda7f513
	https://git.kernel.org/stable/c/ac8b270b61d48fcc61f052097777e3b5e11591e0