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Daniel Sneddon f2f41ef035 x86/speculation: Add RSB VM Exit protections 
commit 2b12993220 upstream.

tl;dr: The Enhanced IBRS mitigation for Spectre v2 does not work as
documented for RET instructions after VM exits. Mitigate it with a new
one-entry RSB stuffing mechanism and a new LFENCE.

== Background ==

Indirect Branch Restricted Speculation (IBRS) was designed to help
mitigate Branch Target Injection and Speculative Store Bypass, i.e.
Spectre, attacks. IBRS prevents software run in less privileged modes
from affecting branch prediction in more privileged modes. IBRS requires
the MSR to be written on every privilege level change.

To overcome some of the performance issues of IBRS, Enhanced IBRS was
introduced.  eIBRS is an "always on" IBRS, in other words, just turn
it on once instead of writing the MSR on every privilege level change.
When eIBRS is enabled, more privileged modes should be protected from
less privileged modes, including protecting VMMs from guests.

== Problem ==

Here's a simplification of how guests are run on Linux' KVM:

void run_kvm_guest(void)
{
	// Prepare to run guest
	VMRESUME();
	// Clean up after guest runs
}

The execution flow for that would look something like this to the
processor:

1. Host-side: call run_kvm_guest()
2. Host-side: VMRESUME
3. Guest runs, does "CALL guest_function"
4. VM exit, host runs again
5. Host might make some "cleanup" function calls
6. Host-side: RET from run_kvm_guest()

Now, when back on the host, there are a couple of possible scenarios of
post-guest activity the host needs to do before executing host code:

* on pre-eIBRS hardware (legacy IBRS, or nothing at all), the RSB is not
touched and Linux has to do a 32-entry stuffing.

* on eIBRS hardware, VM exit with IBRS enabled, or restoring the host
IBRS=1 shortly after VM exit, has a documented side effect of flushing
the RSB except in this PBRSB situation where the software needs to stuff
the last RSB entry "by hand".

IOW, with eIBRS supported, host RET instructions should no longer be
influenced by guest behavior after the host retires a single CALL
instruction.

However, if the RET instructions are "unbalanced" with CALLs after a VM
exit as is the RET in #6, it might speculatively use the address for the
instruction after the CALL in #3 as an RSB prediction. This is a problem
since the (untrusted) guest controls this address.

Balanced CALL/RET instruction pairs such as in step #5 are not affected.

== Solution ==

The PBRSB issue affects a wide variety of Intel processors which
support eIBRS. But not all of them need mitigation. Today,
X86_FEATURE_RETPOLINE triggers an RSB filling sequence that mitigates
PBRSB. Systems setting RETPOLINE need no further mitigation - i.e.,
eIBRS systems which enable retpoline explicitly.

However, such systems (X86_FEATURE_IBRS_ENHANCED) do not set RETPOLINE
and most of them need a new mitigation.

Therefore, introduce a new feature flag X86_FEATURE_RSB_VMEXIT_LITE
which triggers a lighter-weight PBRSB mitigation versus RSB Filling at
vmexit.

The lighter-weight mitigation performs a CALL instruction which is
immediately followed by a speculative execution barrier (INT3). This
steers speculative execution to the barrier -- just like a retpoline
-- which ensures that speculation can never reach an unbalanced RET.
Then, ensure this CALL is retired before continuing execution with an
LFENCE.

In other words, the window of exposure is opened at VM exit where RET
behavior is troublesome. While the window is open, force RSB predictions
sampling for RET targets to a dead end at the INT3. Close the window
with the LFENCE.

There is a subset of eIBRS systems which are not vulnerable to PBRSB.
Add these systems to the cpu_vuln_whitelist[] as NO_EIBRS_PBRSB.
Future systems that aren't vulnerable will set ARCH_CAP_PBRSB_NO.

  [ bp: Massage, incorporate review comments from Andy Cooper. ]
  [ Pawan: Update commit message to replace RSB_VMEXIT with RETPOLINE ]

Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Co-developed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2022-08-11 12:57:53 +02:00
arch x86/speculation: Add RSB VM Exit protections 2022-08-11 12:57:53 +02:00
block block: fix bio_clone_blkg_association() to associate with proper blkcg_gq 2022-06-14 18:11:50 +02:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:11:22 +02:00
crypto crypto: drbg - make reseeding from get_random_bytes() synchronous 2022-06-22 14:11:18 +02:00
Documentation x86/speculation: Add RSB VM Exit protections 2022-08-11 12:57:53 +02:00
drivers macintosh/adb: fix oob read in do_adb_query() function 2022-08-11 12:57:53 +02:00
fs ntfs: fix use-after-free in ntfs_ucsncmp() 2022-08-03 11:59:37 +02:00
include ipv6/addrconf: fix a null-ptr-deref bug for ip6_ptr 2022-08-03 11:59:39 +02:00
init random: handle latent entropy and command line from random_init() 2022-06-22 14:11:17 +02:00
ipc
kernel bpf: Verifer, adjust_scalar_min_max_vals to always call update_reg_bounds() 2022-08-11 12:57:51 +02:00
lib locking/refcount: Consolidate implementations of refcount_t 2022-07-29 17:14:17 +02:00
LICENSES
mm mm/mempolicy: fix uninit-value in mpol_rebind_policy() 2022-07-29 17:14:16 +02:00
net sctp: leave the err path free in sctp_stream_init to sctp_stream_free 2022-08-03 11:59:41 +02:00
samples
scripts modpost: fix section mismatch check for exported init/exit sections 2022-06-29 08:58:49 +02:00
security ima: remove the IMA_TEMPLATE Kconfig option 2022-07-29 17:14:16 +02:00
sound ALSA: memalloc: Align buffer allocations in page size 2022-07-29 17:14:18 +02:00
tools x86/speculation: Add RSB VM Exit protections 2022-08-11 12:57:53 +02:00
usr
virt KVM: Don't null dereference ops->destroy 2022-08-11 12:57:52 +02:00
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MAINTAINERS MAINTAINERS: co-maintain random.c 2022-06-22 14:11:05 +02:00
Makefile Linux 5.4.209 2022-08-03 11:59:42 +02:00
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There are several guides for kernel developers and users. These guides can
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In order to build the documentation, use ``make htmldocs`` or
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There are various text files in the Documentation/ subdirectory,
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Please read the Documentation/process/changes.rst file, as it contains the
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the problems which may result by upgrading your kernel.