* [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state
@ 2018-06-01 14:06 Andrew Cooper
2018-06-01 14:09 ` Andrew Cooper
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Andrew Cooper @ 2018-06-01 14:06 UTC (permalink / raw)
To: Xen-devel; +Cc: Juergen Gross, Andrew Cooper, Jan Beulich
c/s "x86/pv: Introduce and use x86emul_write_dr()" fixed a bug with IO shadow
handling, in that it remained stale and visible until %dr7.L/G got set again.
However, it neglected the -EPERM return inbetween these two hunks, introducing
a different bug in which a write to %dr7 which tries to set IO breakpoints
without %cr4.DE being set clobbers the IO state, rather than leaves it alone.
Instead, move the zeroing slightly later, which guarentees that the shadow
gets written exactly once, on a successful update to %dr7.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Juergen Gross <jgross@suse.com>
Although minor, this is a regression from Xen 4.10, so should be considered
for 4.11 at this point. Given that PV debugging was basically completely
broken before the XSA-263 investigation work, the risk of further issues is
very small.
---
xen/arch/x86/traps.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 8a99174..e79ca88 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2123,9 +2123,6 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
if ( value & DR_GENERAL_DETECT )
return -EPERM;
- /* Zero the IO shadow before recalculating the real %dr7 */
- v->arch.debugreg[5] = 0;
-
/* DR7.{G,L}E = 0 => debugging disabled for this domain. */
if ( value & DR7_ACTIVE_MASK )
{
@@ -2154,6 +2151,10 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
!(v->arch.debugreg[7] & DR7_ACTIVE_MASK) )
activate_debugregs(v);
}
+ else
+ /* Zero the emulated controls if %dr7 isn't active. */
+ v->arch.debugreg[5] = 0;
+
if ( v == curr )
write_debugreg(7, value);
break;
--
2.1.4
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^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state
2018-06-01 14:06 [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state Andrew Cooper
@ 2018-06-01 14:09 ` Andrew Cooper
2018-06-01 15:41 ` Jan Beulich
2018-06-01 18:53 ` Juergen Gross
2 siblings, 0 replies; 4+ messages in thread
From: Andrew Cooper @ 2018-06-01 14:09 UTC (permalink / raw)
To: Xen-devel; +Cc: Juergen Gross, Jan Beulich
On 01/06/18 15:06, Andrew Cooper wrote:
> c/s "x86/pv: Introduce and use x86emul_write_dr()" fixed a bug with IO shadow
> handling, in that it remained stale and visible until %dr7.L/G got set again.
>
> However, it neglected the -EPERM return inbetween these two hunks, introducing
> a different bug in which a write to %dr7 which tries to set IO breakpoints
> without %cr4.DE being set clobbers the IO state, rather than leaves it alone.
>
> Instead, move the zeroing slightly later, which guarentees that the shadow
> gets written exactly once, on a successful update to %dr7.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Juergen Gross <jgross@suse.com>
>
> Although minor, this is a regression from Xen 4.10, so should be considered
> for 4.11 at this point. Given that PV debugging was basically completely
> broken before the XSA-263 investigation work, the risk of further issues is
> very small.
Sorry - this is a bad way of phrasing what I meant to say. The
behaviour of 4.11 is less bad than 4.10, but still wrong (and in a way
which is technically a regression from 4.10).
~Andrew
> ---
> xen/arch/x86/traps.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
> index 8a99174..e79ca88 100644
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -2123,9 +2123,6 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
> if ( value & DR_GENERAL_DETECT )
> return -EPERM;
>
> - /* Zero the IO shadow before recalculating the real %dr7 */
> - v->arch.debugreg[5] = 0;
> -
> /* DR7.{G,L}E = 0 => debugging disabled for this domain. */
> if ( value & DR7_ACTIVE_MASK )
> {
> @@ -2154,6 +2151,10 @@ long set_debugreg(struct vcpu *v, unsigned int reg, unsigned long value)
> !(v->arch.debugreg[7] & DR7_ACTIVE_MASK) )
> activate_debugregs(v);
> }
> + else
> + /* Zero the emulated controls if %dr7 isn't active. */
> + v->arch.debugreg[5] = 0;
> +
> if ( v == curr )
> write_debugreg(7, value);
> break;
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state
2018-06-01 14:06 [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state Andrew Cooper
2018-06-01 14:09 ` Andrew Cooper
@ 2018-06-01 15:41 ` Jan Beulich
2018-06-01 18:53 ` Juergen Gross
2 siblings, 0 replies; 4+ messages in thread
From: Jan Beulich @ 2018-06-01 15:41 UTC (permalink / raw)
To: Andrew Cooper; +Cc: Juergen Gross, Xen-devel
>>> On 01.06.18 at 16:06, <andrew.cooper3@citrix.com> wrote:
> c/s "x86/pv: Introduce and use x86emul_write_dr()" fixed a bug with IO shadow
> handling, in that it remained stale and visible until %dr7.L/G got set again.
>
> However, it neglected the -EPERM return inbetween these two hunks, introducing
> a different bug in which a write to %dr7 which tries to set IO breakpoints
> without %cr4.DE being set clobbers the IO state, rather than leaves it alone.
>
> Instead, move the zeroing slightly later, which guarentees that the shadow
> gets written exactly once, on a successful update to %dr7.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state
2018-06-01 14:06 [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state Andrew Cooper
2018-06-01 14:09 ` Andrew Cooper
2018-06-01 15:41 ` Jan Beulich
@ 2018-06-01 18:53 ` Juergen Gross
2 siblings, 0 replies; 4+ messages in thread
From: Juergen Gross @ 2018-06-01 18:53 UTC (permalink / raw)
To: Andrew Cooper, Xen-devel; +Cc: Jan Beulich
On 01/06/18 16:06, Andrew Cooper wrote:
> c/s "x86/pv: Introduce and use x86emul_write_dr()" fixed a bug with IO shadow
> handling, in that it remained stale and visible until %dr7.L/G got set again.
>
> However, it neglected the -EPERM return inbetween these two hunks, introducing
> a different bug in which a write to %dr7 which tries to set IO breakpoints
> without %cr4.DE being set clobbers the IO state, rather than leaves it alone.
>
> Instead, move the zeroing slightly later, which guarentees that the shadow
> gets written exactly once, on a successful update to %dr7.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Juergen
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^ permalink raw reply [flat|nested] 4+ messages in thread
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2018-06-01 14:06 [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state Andrew Cooper
2018-06-01 14:09 ` Andrew Cooper
2018-06-01 15:41 ` Jan Beulich
2018-06-01 18:53 ` Juergen Gross
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