* [PATCH 0/2] x86/speculation/mds: Minor fixes
@ 2019-05-14 20:24 Andy Lutomirski
2019-05-14 20:24 ` [PATCH 1/2] x86/speculation/mds: Revert CPU buffer clear on double fault exit Andy Lutomirski
2019-05-14 20:24 ` [PATCH 2/2] x86/speculation/mds: Improve CPU buffer clear documentation Andy Lutomirski
0 siblings, 2 replies; 5+ messages in thread
From: Andy Lutomirski @ 2019-05-14 20:24 UTC (permalink / raw)
To: x86; +Cc: LKML, Andy Lutomirski
The first I heard of MDS was today. Let's fix the problems I noticed
right away.
Andy Lutomirski (2):
x86/speculation/mds: Revert CPU buffer clear on double fault exit
x86/speculation/mds: Improve CPU buffer clear documentation
Documentation/x86/mds.rst | 44 ++++++---------------------------------
arch/x86/kernel/traps.c | 8 -------
2 files changed, 6 insertions(+), 46 deletions(-)
--
2.21.0
^ permalink raw reply [flat|nested] 5+ messages in thread
* [PATCH 1/2] x86/speculation/mds: Revert CPU buffer clear on double fault exit
2019-05-14 20:24 [PATCH 0/2] x86/speculation/mds: Minor fixes Andy Lutomirski
@ 2019-05-14 20:24 ` Andy Lutomirski
2019-05-16 7:10 ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
2019-05-14 20:24 ` [PATCH 2/2] x86/speculation/mds: Improve CPU buffer clear documentation Andy Lutomirski
1 sibling, 1 reply; 5+ messages in thread
From: Andy Lutomirski @ 2019-05-14 20:24 UTC (permalink / raw)
To: x86
Cc: LKML, Andy Lutomirski, stable, Greg Kroah-Hartman,
Borislav Petkov, Frederic Weisbecker, Jon Masters
The double fault ESPFIX path doesn't return to user mode at all --
it returns back to the kernel by simulating a #GP fault.
prepare_exit_to_usermode() will run on the way out of
general_protection before running user code.
Cc: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Jon Masters <jcm@redhat.com>
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
Documentation/x86/mds.rst | 7 -------
arch/x86/kernel/traps.c | 8 --------
2 files changed, 15 deletions(-)
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 534e9baa4e1d..0dc812bb9249 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -158,13 +158,6 @@ Mitigation points
mitigated on the return from do_nmi() to provide almost complete
coverage.
- - Double fault (#DF):
-
- A double fault is usually fatal, but the ESPFIX workaround, which can
- be triggered from user space through modify_ldt(2) is a recoverable
- double fault. #DF uses the paranoid exit path, so explicit mitigation
- in the double fault handler is required.
-
- Machine Check Exception (#MC):
Another corner case is a #MC which hits between the CPU buffer clear
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7de466eb960b..8b6d03e55d2f 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -58,7 +58,6 @@
#include <asm/alternative.h>
#include <asm/fpu/xstate.h>
#include <asm/trace/mpx.h>
-#include <asm/nospec-branch.h>
#include <asm/mpx.h>
#include <asm/vm86.h>
#include <asm/umip.h>
@@ -368,13 +367,6 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
regs->ip = (unsigned long)general_protection;
regs->sp = (unsigned long)&gpregs->orig_ax;
- /*
- * This situation can be triggered by userspace via
- * modify_ldt(2) and the return does not take the regular
- * user space exit, so a CPU buffer clear is required when
- * MDS mitigation is enabled.
- */
- mds_user_clear_cpu_buffers();
return;
}
#endif
--
2.21.0
^ permalink raw reply related [flat|nested] 5+ messages in thread
* [PATCH 2/2] x86/speculation/mds: Improve CPU buffer clear documentation
2019-05-14 20:24 [PATCH 0/2] x86/speculation/mds: Minor fixes Andy Lutomirski
2019-05-14 20:24 ` [PATCH 1/2] x86/speculation/mds: Revert CPU buffer clear on double fault exit Andy Lutomirski
@ 2019-05-14 20:24 ` Andy Lutomirski
2019-05-16 7:11 ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
1 sibling, 1 reply; 5+ messages in thread
From: Andy Lutomirski @ 2019-05-14 20:24 UTC (permalink / raw)
To: x86
Cc: LKML, Andy Lutomirski, stable, Greg Kroah-Hartman,
Borislav Petkov, Frederic Weisbecker, Jon Masters
On x86_64, all returns to usermode go through
prepare_exit_to_usermode(), with the sole exception of do_nmi().
This even includes machine checks -- this was added several years
ago to support MCE recovery. Update the documentation.
Cc: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Jon Masters <jcm@redhat.com>
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
Documentation/x86/mds.rst | 39 +++++++--------------------------------
1 file changed, 7 insertions(+), 32 deletions(-)
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 0dc812bb9249..5d4330be200f 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -142,38 +142,13 @@ Mitigation points
mds_user_clear.
The mitigation is invoked in prepare_exit_to_usermode() which covers
- most of the kernel to user space transitions. There are a few exceptions
- which are not invoking prepare_exit_to_usermode() on return to user
- space. These exceptions use the paranoid exit code.
-
- - Non Maskable Interrupt (NMI):
-
- Access to sensible data like keys, credentials in the NMI context is
- mostly theoretical: The CPU can do prefetching or execute a
- misspeculated code path and thereby fetching data which might end up
- leaking through a buffer.
-
- But for mounting other attacks the kernel stack address of the task is
- already valuable information. So in full mitigation mode, the NMI is
- mitigated on the return from do_nmi() to provide almost complete
- coverage.
-
- - Machine Check Exception (#MC):
-
- Another corner case is a #MC which hits between the CPU buffer clear
- invocation and the actual return to user. As this still is in kernel
- space it takes the paranoid exit path which does not clear the CPU
- buffers. So the #MC handler repopulates the buffers to some
- extent. Machine checks are not reliably controllable and the window is
- extremly small so mitigation would just tick a checkbox that this
- theoretical corner case is covered. To keep the amount of special
- cases small, ignore #MC.
-
- - Debug Exception (#DB):
-
- This takes the paranoid exit path only when the INT1 breakpoint is in
- kernel space. #DB on a user space address takes the regular exit path,
- so no extra mitigation required.
+ all but one of the kernel to user space transitions. The exception
+ is when we return from a Non Maskable Interrupt (NMI), which is
+ handled directly in do_nmi().
+
+ (The reason that NMI is special is that prepare_exit_to_usermode() can
+ enable IRQs. In NMI context, NMIs are blocked, and we don't want to
+ enable IRQs with NMIs blocked.)
2. C-State transition
--
2.21.0
^ permalink raw reply related [flat|nested] 5+ messages in thread
* [tip:x86/urgent] x86/speculation/mds: Revert CPU buffer clear on double fault exit
2019-05-14 20:24 ` [PATCH 1/2] x86/speculation/mds: Revert CPU buffer clear on double fault exit Andy Lutomirski
@ 2019-05-16 7:10 ` tip-bot for Andy Lutomirski
0 siblings, 0 replies; 5+ messages in thread
From: tip-bot for Andy Lutomirski @ 2019-05-16 7:10 UTC (permalink / raw)
To: linux-tip-commits
Cc: tglx, gregkh, linux-kernel, frederic, bp, peterz, torvalds,
mingo, jcm, hpa, luto
Commit-ID: 88640e1dcd089879530a49a8d212d1814678dfe7
Gitweb: https://git.kernel.org/tip/88640e1dcd089879530a49a8d212d1814678dfe7
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 14 May 2019 13:24:39 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 16 May 2019 09:05:11 +0200
x86/speculation/mds: Revert CPU buffer clear on double fault exit
The double fault ESPFIX path doesn't return to user mode at all --
it returns back to the kernel by simulating a #GP fault.
prepare_exit_to_usermode() will run on the way out of
general_protection before running user code.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jon Masters <jcm@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Link: http://lkml.kernel.org/r/ac97612445c0a44ee10374f6ea79c222fe22a5c4.1557865329.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
Documentation/x86/mds.rst | 7 -------
arch/x86/kernel/traps.c | 8 --------
2 files changed, 15 deletions(-)
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 534e9baa4e1d..0dc812bb9249 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -158,13 +158,6 @@ Mitigation points
mitigated on the return from do_nmi() to provide almost complete
coverage.
- - Double fault (#DF):
-
- A double fault is usually fatal, but the ESPFIX workaround, which can
- be triggered from user space through modify_ldt(2) is a recoverable
- double fault. #DF uses the paranoid exit path, so explicit mitigation
- in the double fault handler is required.
-
- Machine Check Exception (#MC):
Another corner case is a #MC which hits between the CPU buffer clear
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7de466eb960b..8b6d03e55d2f 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -58,7 +58,6 @@
#include <asm/alternative.h>
#include <asm/fpu/xstate.h>
#include <asm/trace/mpx.h>
-#include <asm/nospec-branch.h>
#include <asm/mpx.h>
#include <asm/vm86.h>
#include <asm/umip.h>
@@ -368,13 +367,6 @@ dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
regs->ip = (unsigned long)general_protection;
regs->sp = (unsigned long)&gpregs->orig_ax;
- /*
- * This situation can be triggered by userspace via
- * modify_ldt(2) and the return does not take the regular
- * user space exit, so a CPU buffer clear is required when
- * MDS mitigation is enabled.
- */
- mds_user_clear_cpu_buffers();
return;
}
#endif
^ permalink raw reply related [flat|nested] 5+ messages in thread
* [tip:x86/urgent] x86/speculation/mds: Improve CPU buffer clear documentation
2019-05-14 20:24 ` [PATCH 2/2] x86/speculation/mds: Improve CPU buffer clear documentation Andy Lutomirski
@ 2019-05-16 7:11 ` tip-bot for Andy Lutomirski
0 siblings, 0 replies; 5+ messages in thread
From: tip-bot for Andy Lutomirski @ 2019-05-16 7:11 UTC (permalink / raw)
To: linux-tip-commits
Cc: mingo, tglx, hpa, torvalds, linux-kernel, frederic, bp, luto,
gregkh, peterz, jcm
Commit-ID: 9d8d0294e78a164d407133dea05caf4b84247d6a
Gitweb: https://git.kernel.org/tip/9d8d0294e78a164d407133dea05caf4b84247d6a
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 14 May 2019 13:24:40 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 16 May 2019 09:05:12 +0200
x86/speculation/mds: Improve CPU buffer clear documentation
On x86_64, all returns to usermode go through
prepare_exit_to_usermode(), with the sole exception of do_nmi().
This even includes machine checks -- this was added several years
ago to support MCE recovery. Update the documentation.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jon Masters <jcm@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: 04dcbdb80578 ("x86/speculation/mds: Clear CPU buffers on exit to user")
Link: http://lkml.kernel.org/r/999fa9e126ba6a48e9d214d2f18dbde5c62ac55c.1557865329.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
Documentation/x86/mds.rst | 39 +++++++--------------------------------
1 file changed, 7 insertions(+), 32 deletions(-)
diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst
index 0dc812bb9249..5d4330be200f 100644
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -142,38 +142,13 @@ Mitigation points
mds_user_clear.
The mitigation is invoked in prepare_exit_to_usermode() which covers
- most of the kernel to user space transitions. There are a few exceptions
- which are not invoking prepare_exit_to_usermode() on return to user
- space. These exceptions use the paranoid exit code.
-
- - Non Maskable Interrupt (NMI):
-
- Access to sensible data like keys, credentials in the NMI context is
- mostly theoretical: The CPU can do prefetching or execute a
- misspeculated code path and thereby fetching data which might end up
- leaking through a buffer.
-
- But for mounting other attacks the kernel stack address of the task is
- already valuable information. So in full mitigation mode, the NMI is
- mitigated on the return from do_nmi() to provide almost complete
- coverage.
-
- - Machine Check Exception (#MC):
-
- Another corner case is a #MC which hits between the CPU buffer clear
- invocation and the actual return to user. As this still is in kernel
- space it takes the paranoid exit path which does not clear the CPU
- buffers. So the #MC handler repopulates the buffers to some
- extent. Machine checks are not reliably controllable and the window is
- extremly small so mitigation would just tick a checkbox that this
- theoretical corner case is covered. To keep the amount of special
- cases small, ignore #MC.
-
- - Debug Exception (#DB):
-
- This takes the paranoid exit path only when the INT1 breakpoint is in
- kernel space. #DB on a user space address takes the regular exit path,
- so no extra mitigation required.
+ all but one of the kernel to user space transitions. The exception
+ is when we return from a Non Maskable Interrupt (NMI), which is
+ handled directly in do_nmi().
+
+ (The reason that NMI is special is that prepare_exit_to_usermode() can
+ enable IRQs. In NMI context, NMIs are blocked, and we don't want to
+ enable IRQs with NMIs blocked.)
2. C-State transition
^ permalink raw reply related [flat|nested] 5+ messages in thread
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2019-05-14 20:24 [PATCH 0/2] x86/speculation/mds: Minor fixes Andy Lutomirski
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2019-05-16 7:10 ` [tip:x86/urgent] " tip-bot for Andy Lutomirski
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