* Re: A possible pointer_overflow in xen-4.13
@ 2021-07-07 15:54 =?gb18030?B?UnJvYWNo?=
2021-07-07 15:59 ` Jan Beulich
0 siblings, 1 reply; 5+ messages in thread
From: =?gb18030?B?UnJvYWNo?= @ 2021-07-07 15:54 UTC (permalink / raw)
To: =?gb18030?B?SmFuIEJldWxpY2g=?=, =?gb18030?B?eGVuLWRldmVs?=
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Hi, sorry about the late respond. I tried your suggestion, it works. I'm kind of surprised too, since such problem should exposed long time ago.
I looked deep into your suggestion. I believe you were right about it, since p - ctxt->io_emul_stub won't overflow and the pointer overflow is likely to happen in stub_va + p or ctxt->io_emul_stub.
Andrew's suggestion works perhaps it the long variable allows the expression to store more bytes, however in long term it may not be a solid solution. So alternative should we take both of the advise that using
+ long disp = (long)(f) - (long)(stub_va + (p - ctxt->io_emul_stub) + 5); \
as a fix patch
Best regards
Franklin
------------------ Original ------------------
From: "Jan Beulich";<jbeulich@suse.com>;
Send time: Wednesday, Jul 7, 2021 3:55 PM
To: "Rroach"<2284696125@qq.com>;
Cc: "xen-devel"<xen-devel@lists.xenproject.org>; "Andrew Cooper"<andrew.cooper3@citrix.com>;
Subject: Re: A possible pointer_overflow in xen-4.13
On 07.07.2021 04:32, Rroach wrote:
> After patching it, this works fine and UBSAN dose not have any error report about it.
Which I'm surprised about, because in Andrew's suggestion (sorry,
need to reproduce it manually, because quoting your HTML mail is
rendering unreadable results, as you can see below if you view it
as plain text)
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
#define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
#define APPEND_CALL(f) \
({ \
- long disp = (long)(f) - (stub_va + p - ctxt->io_emul_stub + 5); \
+ long disp = (long)(f) - (long)(stub_va + p - ctxt->io_emul_stub + 5); \
there is still a possible pointer overflow afaict, unlike in the
suggestion I had given:
long disp = (long)(f) - (stub_va + (p - ctxt->io_emul_stub) + 5); \
This because of expression evaluation order, which I understand would
match the fully parenthesized
long disp = (long)(f) - (long)(((stub_va + p) - ctxt->io_emul_stub) + 5); \
where both stub_va + p and the subsequent subtraction of ctxt->io_emul_stub
are liable to overflow. Whereas p - ctxt->io_emul_stub won't overflow, as
p starts out from ctxt->io_emul_stub and then gets incremented by a few bytes.
Would you mind giving the alternative suggestion a try as well?
Jan
> ------------------&nbsp;Original&nbsp;------------------
> From: &nbsp;"Andrew Cooper";<andrew.cooper3@citrix.com&gt;;
> Send time:&nbsp;Saturday, Jun 26, 2021 9:50 PM
> To:&nbsp;"Rroach"<2284696125@qq.com&gt;; "xen-devel"<xen-devel@lists.xenproject.org&gt;;
>
> Subject: &nbsp;Re: A possible pointer_overflow in xen-4.13
>
>
>
> On 26/06/2021 14:29, Rroach wrote:
>
> Hi, I compile Xen-4.13 with CONFIG_UBSAN, and try test it. However, during testing, xl dmesg got the output as shown below.
>
>
> It seems that there is a potential pointer overflow within arch/x86/pv/emul-priv-op.c:131 where xen try to execute instruction ''' APPEND_CALL(save_guest_gprs) '''£¬where APPEND_CALL try to add an offset on *p without proper checking.
>
>
> I compiled xen-4.13 by clang-9, with following instructions: ''' export CONFIG_UBSAN=y ''' &amp;&amp; ''' make clang=y debug=y ''' . Do you have any idea what going on here?
>
>
> You say Xen 4.13, but APPEND_CALL() doesn't exist there.&nbsp; I added it in 4.14 when I rewrote this mess to be compatible with CET by not using a ROP gadget.&nbsp; Your backtrace says 4.15 unstable which means its an old staging build (not that that is going to have any effect on this specific issue).
>
> The fact that it continued executing correctly means the calculation did the right thing, whether or not UBSAN was happy. The displacement will end up negative as the stub we're writing is numerically higher than {load,save}_guest_gprs(), which I guess means that f - stub_va will underflow.
>
> I'm very confused as to why UBSAN reports against save_guest_gprs() considering that load_guest_gprs() when through the exact same logic a few instructions earlier.
>
> Either way, does this make the problem go away?
>
> diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
> index 11467a1e3a..be41bced76 100644
> --- a/xen/arch/x86/pv/emul-priv-op.c
> +++ b/xen/arch/x86/pv/emul-priv-op.c
> @@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
> &nbsp;#define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
> &nbsp;#define APPEND_CALL(f)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
> &nbsp;&nbsp;&nbsp; ({&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; \
> -&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; long disp = (long)(f) - (stub_va + p - ctxt-&gt;io_emul_stub + 5); \
> +&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; long disp = (long)(f) - (long)(stub_va + p - ctxt-&gt;io_emul_stub + 5); \
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; BUG_ON((int32_t)disp != disp);&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; *p++ = 0xe8;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; *(int32_t *)p = disp; p += 4;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; \
>
> ~Andrew
>
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^ permalink raw reply [flat|nested] 5+ messages in thread* Re: A possible pointer_overflow in xen-4.13
2021-07-07 15:54 A possible pointer_overflow in xen-4.13 =?gb18030?B?UnJvYWNo?=
@ 2021-07-07 15:59 ` Jan Beulich
0 siblings, 0 replies; 5+ messages in thread
From: Jan Beulich @ 2021-07-07 15:59 UTC (permalink / raw)
To: Rroach; +Cc: xen-devel
On 07.07.2021 17:54, Rroach wrote:
> Hi, sorry about the late respond. I tried your suggestion, it works. I'm kind of surprised too, since such problem should exposed long time ago.
>
>
> I looked deep into your suggestion. I believe you were right about it, since p - ctxt->io_emul_stub won't overflow and the pointer overflow is likely to happen in stub_va + p or ctxt->io_emul_stub.
>
>
> Andrew's suggestion works perhaps it the long variable allows the expression to store more bytes,
Xen (as much as e.g. Linux and I think most other Unix-es) assumes sizeof(void*)
and sizeof(long) to be the same.
> however in long term it may not be a solid solution. So alternative should we take both of the advise that using
> + long disp = (long)(f) - (long)(stub_va + (p - ctxt->io_emul_stub) + 5); \
> as a fix patch
I don't think so - we try to avoid casts wherever they're not strictly needed.
Btw, to record you in an eventual patch with a Reported-by, would you mind
providing your real name and maybe a less temporary-looking email address?
Jan
^ permalink raw reply [flat|nested] 5+ messages in thread
[parent not found: <tencent_A17CA7BA63F6E47B3FE7B1AC54E55B2A3609@qq.com>]
* Re: A possible pointer_overflow in xen-4.13
[not found] <tencent_A17CA7BA63F6E47B3FE7B1AC54E55B2A3609@qq.com>
@ 2021-06-26 13:50 ` Andrew Cooper
2021-07-07 2:32 ` =?gb18030?B?UnJvYWNo?=
0 siblings, 1 reply; 5+ messages in thread
From: Andrew Cooper @ 2021-06-26 13:50 UTC (permalink / raw)
To: Rroach, xen-devel
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On 26/06/2021 14:29, Rroach wrote:
> Hi, I compile Xen-4.13 with CONFIG_UBSAN, and try test it. However,
> during testing, xl dmesg got the output as shown below.
>
> It seems that there is a potential pointer overflow within
> arch/x86/pv/emul-priv-op.c:131 where xen try to execute instruction
> ''' APPEND_CALL(save_guest_gprs) '''£¬where APPEND_CALL try to add an
> offset on *p without proper checking.
>
> I compiled xen-4.13 by clang-9, with following instructions: '''
> export CONFIG_UBSAN=y ''' && ''' make clang=y debug=y ''' . Do you
> have any idea what going on here?
You say Xen 4.13, but APPEND_CALL() doesn't exist there.0„2 I added it in
4.14 when I rewrote this mess to be compatible with CET by not using a
ROP gadget.0„2 Your backtrace says 4.15 unstable which means its an old
staging build (not that that is going to have any effect on this
specific issue).
The fact that it continued executing correctly means the calculation did
the right thing, whether or not UBSAN was happy.0„2 The displacement will
end up negative as the stub we're writing is numerically higher than
{load,save}_guest_gprs(), which I guess means that f - stub_va will
underflow.
I'm very confused as to why UBSAN reports against save_guest_gprs()
considering that load_guest_gprs() when through the exact same logic a
few instructions earlier.
Either way, does this make the problem go away?
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 11467a1e3a..be41bced76 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct
priv_op_ctxt *ctxt, u8 opcode,
0„2#define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
0„2#define APPEND_CALL(f)0„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„2 \
0„20„20„20„2 ({0„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„2 \
-0„20„20„20„20„20„20„2 long disp = (long)(f) - (stub_va + p - ctxt->io_emul_stub + 5); \
+0„20„20„20„20„20„20„2 long disp = (long)(f) - (long)(stub_va + p - ctxt->io_emul_stub
+ 5); \
0„20„20„20„20„20„20„20„2 BUG_ON((int32_t)disp != disp);0„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„2 \
0„20„20„20„20„20„20„20„2 *p++ = 0xe8;0„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„2 \
0„20„20„20„20„20„20„20„2 *(int32_t *)p = disp; p += 4;0„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„20„2 \
~Andrew
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^ permalink raw reply related [flat|nested] 5+ messages in thread* Re: A possible pointer_overflow in xen-4.13
2021-06-26 13:50 ` Andrew Cooper
@ 2021-07-07 2:32 ` =?gb18030?B?UnJvYWNo?=
2021-07-07 7:55 ` Jan Beulich
0 siblings, 1 reply; 5+ messages in thread
From: =?gb18030?B?UnJvYWNo?= @ 2021-07-07 2:32 UTC (permalink / raw)
To: =?gb18030?B?eGVuLWRldmVs?=
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After patching it, this works fine and UBSAN dose not have any error report about it.
------------------ Original ------------------
From: "Andrew Cooper";<andrew.cooper3@citrix.com>;
Send time: Saturday, Jun 26, 2021 9:50 PM
To: "Rroach"<2284696125@qq.com>; "xen-devel"<xen-devel@lists.xenproject.org>;
Subject: Re: A possible pointer_overflow in xen-4.13
On 26/06/2021 14:29, Rroach wrote:
Hi, I compile Xen-4.13 with CONFIG_UBSAN, and try test it. However, during testing, xl dmesg got the output as shown below.
It seems that there is a potential pointer overflow within arch/x86/pv/emul-priv-op.c:131 where xen try to execute instruction ''' APPEND_CALL(save_guest_gprs) '''£¬where APPEND_CALL try to add an offset on *p without proper checking.
I compiled xen-4.13 by clang-9, with following instructions: ''' export CONFIG_UBSAN=y ''' && ''' make clang=y debug=y ''' . Do you have any idea what going on here?
You say Xen 4.13, but APPEND_CALL() doesn't exist there. I added it in 4.14 when I rewrote this mess to be compatible with CET by not using a ROP gadget. Your backtrace says 4.15 unstable which means its an old staging build (not that that is going to have any effect on this specific issue).
The fact that it continued executing correctly means the calculation did the right thing, whether or not UBSAN was happy. The displacement will end up negative as the stub we're writing is numerically higher than {load,save}_guest_gprs(), which I guess means that f - stub_va will underflow.
I'm very confused as to why UBSAN reports against save_guest_gprs() considering that load_guest_gprs() when through the exact same logic a few instructions earlier.
Either way, does this make the problem go away?
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index 11467a1e3a..be41bced76 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
#define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
#define APPEND_CALL(f) \
({ \
- long disp = (long)(f) - (stub_va + p - ctxt->io_emul_stub + 5); \
+ long disp = (long)(f) - (long)(stub_va + p - ctxt->io_emul_stub + 5); \
BUG_ON((int32_t)disp != disp); \
*p++ = 0xe8; \
*(int32_t *)p = disp; p += 4; \
~Andrew
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^ permalink raw reply [flat|nested] 5+ messages in thread* Re: A possible pointer_overflow in xen-4.13
2021-07-07 2:32 ` =?gb18030?B?UnJvYWNo?=
@ 2021-07-07 7:55 ` Jan Beulich
0 siblings, 0 replies; 5+ messages in thread
From: Jan Beulich @ 2021-07-07 7:55 UTC (permalink / raw)
To: Rroach; +Cc: xen-devel, Andrew Cooper
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On 07.07.2021 04:32, Rroach wrote:
> After patching it, this works fine and UBSAN dose not have any error report about it.
Which I'm surprised about, because in Andrew's suggestion (sorry,
need to reproduce it manually, because quoting your HTML mail is
rendering unreadable results, as you can see below if you view it
as plain text)
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
#define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
#define APPEND_CALL(f) \
({ \
- long disp = (long)(f) - (stub_va + p - ctxt->io_emul_stub + 5); \
+ long disp = (long)(f) - (long)(stub_va + p - ctxt->io_emul_stub + 5); \
there is still a possible pointer overflow afaict, unlike in the
suggestion I had given:
long disp = (long)(f) - (stub_va + (p - ctxt->io_emul_stub) + 5); \
This because of expression evaluation order, which I understand would
match the fully parenthesized
long disp = (long)(f) - (long)(((stub_va + p) - ctxt->io_emul_stub) + 5); \
where both stub_va + p and the subsequent subtraction of ctxt->io_emul_stub
are liable to overflow. Whereas p - ctxt->io_emul_stub won't overflow, as
p starts out from ctxt->io_emul_stub and then gets incremented by a few bytes.
Would you mind giving the alternative suggestion a try as well?
Jan
> ------------------ Original ------------------
> From: "Andrew Cooper";<andrew.cooper3@citrix.com>;
> Send time: Saturday, Jun 26, 2021 9:50 PM
> To: "Rroach"<2284696125@qq.com>; "xen-devel"<xen-devel@lists.xenproject.org>;
>
> Subject: Re: A possible pointer_overflow in xen-4.13
>
>
>
> On 26/06/2021 14:29, Rroach wrote:
>
> Hi, I compile Xen-4.13 with CONFIG_UBSAN, and try test it. However, during testing, xl dmesg got the output as shown below.
>
>
> It seems that there is a potential pointer overflow within arch/x86/pv/emul-priv-op.c:131 where xen try to execute instruction ''' APPEND_CALL(save_guest_gprs) '''£¬where APPEND_CALL try to add an offset on *p without proper checking.
>
>
> I compiled xen-4.13 by clang-9, with following instructions: ''' export CONFIG_UBSAN=y ''' && ''' make clang=y debug=y ''' . Do you have any idea what going on here?
>
>
> You say Xen 4.13, but APPEND_CALL() doesn't exist there. I added it in 4.14 when I rewrote this mess to be compatible with CET by not using a ROP gadget. Your backtrace says 4.15 unstable which means its an old staging build (not that that is going to have any effect on this specific issue).
>
> The fact that it continued executing correctly means the calculation did the right thing, whether or not UBSAN was happy. The displacement will end up negative as the stub we're writing is numerically higher than {load,save}_guest_gprs(), which I guess means that f - stub_va will underflow.
>
> I'm very confused as to why UBSAN reports against save_guest_gprs() considering that load_guest_gprs() when through the exact same logic a few instructions earlier.
>
> Either way, does this make the problem go away?
>
> diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
> index 11467a1e3a..be41bced76 100644
> --- a/xen/arch/x86/pv/emul-priv-op.c
> +++ b/xen/arch/x86/pv/emul-priv-op.c
> @@ -98,7 +98,7 @@ static io_emul_stub_t *io_emul_stub_setup(struct priv_op_ctxt *ctxt, u8 opcode,
> #define APPEND_BUFF(b) ({ memcpy(p, b, sizeof(b)); p += sizeof(b); })
> #define APPEND_CALL(f) \
> ({ \
> - long disp = (long)(f) - (stub_va + p - ctxt->io_emul_stub + 5); \
> + long disp = (long)(f) - (long)(stub_va + p - ctxt->io_emul_stub + 5); \
> BUG_ON((int32_t)disp != disp); \
> *p++ = 0xe8; \
> *(int32_t *)p = disp; p += 4; \
>
> ~Andrew
>
^ permalink raw reply [flat|nested] 5+ messages in thread
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2021-07-07 15:54 A possible pointer_overflow in xen-4.13 =?gb18030?B?UnJvYWNo?=
2021-07-07 15:59 ` Jan Beulich
[not found] <tencent_A17CA7BA63F6E47B3FE7B1AC54E55B2A3609@qq.com>
2021-06-26 13:50 ` Andrew Cooper
2021-07-07 2:32 ` =?gb18030?B?UnJvYWNo?=
2021-07-07 7:55 ` Jan Beulich
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