x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key

The VERW mitigation at exit-to-user is enabled via a static branch
mds_user_clear. This static branch is never toggled after boot, and can
be safely replaced with an ALTERNATIVE() which is convenient to use in
asm.

Switch to ALTERNATIVE() to use the VERW mitigation late in exit-to-user
path. Also remove the now redundant VERW in exc_nmi() and
arch_exit_to_user_mode().

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20240213-delay-verw-v8-4-a6216d83edb7%40linux.intel.com

x86/CPU/AMD: Fix the DIV(0) initial fix attempt

Initially, it was thought that doing an innocuous division in the #DE
handler would take care to prevent any leaking of old data from the
divider but by the time the fault is raised, the speculation has already
advanced too far and such data could already have been used by younger
operations.

Therefore, do the innocuous division on every exit to userspace so that
userspace doesn't see any potentially old data from integer divisions in
kernel space.

Do the same before VMRUN too, to protect host data from leaking into the
guest too.

Fixes: 77245f1c3c64 ("x86/CPU/AMD: Do not leak quotient data after a division by 0")
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20230811213824.10025-1-bp@alien8.de

x86/cpu: Remove unneeded 64-bit dependency in arch_enter_from_user_mode()

The check for 64-bit mode when testing X86_FEATURE_XENPV isn't needed,
as Xen PV guests are no longer supported in 32-bit mode, see

a13f2ef168cb ("x86/xen: remove 32-bit Xen PV guest support").

While at it switch from boot_cpu_has() to cpu_feature_enabled().

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20221104072701.20283-3-jgross@suse.com

entry: Rename arch_check_user_regs() to arch_enter_from_user_mode()

arch_check_user_regs() is used at the moment to verify that struct pt_regs
contains valid values when entering the kernel from userspace. s390 needs
a place in the generic entry code to modify a cpu data structure when
switching from userspace to kernel mode. As arch_check_user_regs() is
exactly this, rename it to arch_enter_from_user_mode().

When entering the kernel from userspace, arch_check_user_regs() is
used to verify that struct pt_regs contains valid values. Note that
the NMI codepath doesn't call this function. s390 needs a place in the
generic entry code to modify a cpu data structure when switching from
userspace to kernel mode. As arch_check_user_regs() is exactly this,
rename it to arch_enter_from_user_mode().

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220504062351.2954280-2-tmricht@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

x86/entry: Correct reference to intended CONFIG_64_BIT

Commit in Fixes adds a condition with IS_ENABLED(CONFIG_64_BIT),
but the intended config item is called CONFIG_64BIT, as defined in
arch/x86/Kconfig.

Fortunately, scripts/checkkconfigsymbols.py warns:

64_BIT
Referencing files: arch/x86/include/asm/entry-common.h

Correct the reference to the intended config symbol.

Fixes: 662a0221893a ("x86/entry: Fix AC assertion")
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20210803113531.30720-2-lukas.bulwahn@gmail.com

x86/entry: Enable random_kstack_offset support

Allow for a randomized stack offset on a per-syscall basis, with roughly
5-6 bits of entropy, depending on compiler and word size. Since the
method of offsetting uses macros, this cannot live in the common entry
code (the stack offset needs to be retained for the life of the syscall,
which means it needs to happen at the actual entry point).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210401232347.2791257-5-keescook@chromium.org

entry: Ensure trap after single-step on system call return

Commit 299155244770 ("entry: Drop usage of TIF flags in the generic syscall
code") introduced a bug on architectures using the generic syscall entry
code, in which processes stopped by PTRACE_SYSCALL do not trap on syscall
return after receiving a TIF_SINGLESTEP.

The reason is that the meaning of TIF_SINGLESTEP flag is overloaded to
cause the trap after a system call is executed, but since the above commit,
the syscall call handler only checks for the SYSCALL_WORK flags on the exit
work.

Split the meaning of TIF_SINGLESTEP such that it only means single-step
mode, and create a new type of SYSCALL_WORK to request a trap immediately
after a syscall in single-step mode. In the current implementation, the
SYSCALL_WORK flag shadows the TIF_SINGLESTEP flag for simplicity.

Update x86 to flip this bit when a tracer enables single stepping.

Fixes: 299155244770 ("entry: Drop usage of TIF flags in the generic syscall code")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kyle Huey <me@kylehuey.com>
Link: https://lore.kernel.org/r/87h7mtc9pr.fsf_-_@collabora.com

x86/entry: Fix AC assertion

The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
improve user entry sanity checks") unconditionally triggers on a IVB
machine because it does not support SMAP.

For !SMAP hardware the CLAC/STAC instructions are patched out and thus if
userspace sets AC, it is still have set after entry.

Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200902133200.666781610@infradead.org

x86/entry: Use generic syscall exit functionality

Replace the x86 variant with the generic version. Provide the relevant
architecture specific helper functions and defines.

Use a temporary define for idtentry_exit_user which will be cleaned up
seperately.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200722220520.494648601@linutronix.de

x86/entry: Use generic syscall entry function

Replace the syscall entry work handling with the generic version. Provide
the necessary helper inlines to handle the real architecture specific
parts, e.g. ptrace.

Use a temporary define for idtentry_enter_user which will be cleaned up
seperately.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20200722220520.376213694@linutronix.de