CVE request: musl libc 1.1.23 and earlier x87 float stack imbalance

CVE request: musl libc 1.1.23 and earlier x87 float stack imbalance

[Rich Felker]

I've discovered a flaw in musl libc's arch-specific math assembly code
for i386, whereby at least the log1p function and possibly others
return with more than one item on the x87 stack. This can lead to x87
stack overflow in the execution of subsequent math code, causing it to
incorrectly produce a NAN in place of the actual result. If floating
point results are used in flow control, this can lead to runaway wrong
code execution. For example, in Python (version 3.6.8 tested), at
least one code path of the dtoa function becomes an infinite loop
performing what's effectively an unbounded-length memset when entered
under such a condition.

This bug is potentially exploitable in software which calls affected
math functions with inputs under user control. Impact depends on how
the application handles the ABI-violating x87 state; in Python it
seems to be limited to producing a crash.

The bug is present in all versions after 0.9.12, up through the
current (1.1.23) release. Only 32-bit x86 systems (aka IA32, musl's
"i386" arch) are affected. Users of other archs, including x86_64, can
safely ignore this issue.

Affected users are advised to apply the following patch:

https://git.musl-libc.org/cgit/musl/patch/?id=f3ed8bfe8a82af1870ddc8696ed4cc1d5aa6b441

Re: CVE request: musl libc 1.1.23 and earlier x87 float stack imbalance

[Rich Felker]

[-- Attachment #1: Type: text/plain, Size: 1748 bytes --]

On Mon, Aug 05, 2019 at 07:27:37PM -0400, Rich Felker wrote:
> I've discovered a flaw in musl libc's arch-specific math assembly code
> for i386, whereby at least the log1p function and possibly others
> return with more than one item on the x87 stack. This can lead to x87
> stack overflow in the execution of subsequent math code, causing it to
> incorrectly produce a NAN in place of the actual result. If floating
> point results are used in flow control, this can lead to runaway wrong
> code execution. For example, in Python (version 3.6.8 tested), at
> least one code path of the dtoa function becomes an infinite loop
> performing what's effectively an unbounded-length memset when entered
> under such a condition.
> 
> This bug is potentially exploitable in software which calls affected
> math functions with inputs under user control. Impact depends on how
> the application handles the ABI-violating x87 state; in Python it
> seems to be limited to producing a crash.
> 
> The bug is present in all versions after 0.9.12, up through the
> current (1.1.23) release. Only 32-bit x86 systems (aka IA32, musl's
> "i386" arch) are affected. Users of other archs, including x86_64, can
> safely ignore this issue.
> 
> Affected users are advised to apply the following patch:
> 
> https://git.musl-libc.org/cgit/musl/patch/?id=f3ed8bfe8a82af1870ddc8696ed4cc1d5aa6b441

The patch contains an error that was missed for unknown reasons,
probably failure to rebuild a file. I'm attaching an aggregate patch
that works. Alternaatively, these two commits can be applied:

https://git.musl-libc.org/cgit/musl/patch/?id=f3ed8bfe8a82af1870ddc8696ed4cc1d5aa6b441
https://git.musl-libc.org/cgit/musl/patch/?id=6818c31c9bc4bbad5357f1de14bedf781e5b349e

[-- Attachment #2: x87_stack_bug.diff --]
[-- Type: text/plain, Size: 3104 bytes --]

diff --git a/src/math/i386/asin.s b/src/math/i386/asin.s
index a9f691bf..920d967a 100644
--- a/src/math/i386/asin.s
+++ b/src/math/i386/asin.s
@@ -7,13 +7,10 @@ asinf:
 	cmp $0x01000000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-2:	ret
+	ret
 
 .global asinl
 .type asinl,@function
@@ -30,11 +27,8 @@ asin:
 	cmp $0x00200000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fsts 4(%esp)
-2:	ret
+	ret
 1:	fld %st(0)
 	fld1
 	fsub %st(0),%st(1)
diff --git a/src/math/i386/atan.s b/src/math/i386/atan.s
index d73137b2..a26feae1 100644
--- a/src/math/i386/atan.s
+++ b/src/math/i386/atan.s
@@ -10,8 +10,5 @@ atan:
 	fpatan
 	ret
 		# subnormal x, return x with underflow
-1:	fnstsw %ax
-	and $16,%ax
-	jnz 2f
-	fsts 4(%esp)
-2:	ret
+1:	fsts 4(%esp)
+	ret
diff --git a/src/math/i386/atan2.s b/src/math/i386/atan2.s
index a7d2979b..76b95f31 100644
--- a/src/math/i386/atan2.s
+++ b/src/math/i386/atan2.s
@@ -10,8 +10,5 @@ atan2:
 	cmp $0x00200000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 1f
 	fsts 4(%esp)
 1:	ret
diff --git a/src/math/i386/atan2f.s b/src/math/i386/atan2f.s
index 14b88ce5..c9408a90 100644
--- a/src/math/i386/atan2f.s
+++ b/src/math/i386/atan2f.s
@@ -10,9 +10,6 @@ atan2f:
 	cmp $0x01000000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 1f
 	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
diff --git a/src/math/i386/atanf.s b/src/math/i386/atanf.s
index 8caddefa..893beac5 100644
--- a/src/math/i386/atanf.s
+++ b/src/math/i386/atanf.s
@@ -10,10 +10,7 @@ atanf:
 	fpatan
 	ret
 		# subnormal x, return x with underflow
-1:	fnstsw %ax
-	and $16,%ax
-	jnz 2f
-	fld %st(0)
+1:	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-2:	ret
+	ret
diff --git a/src/math/i386/exp.s b/src/math/i386/exp.s
index c7aa5b6e..df87c497 100644
--- a/src/math/i386/exp.s
+++ b/src/math/i386/exp.s
@@ -7,13 +7,10 @@ expm1f:
 	cmp $0x01000000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-2:	ret
+	ret
 
 .global expm1l
 .type expm1l,@function
@@ -30,11 +27,8 @@ expm1:
 	cmp $0x00200000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fsts 4(%esp)
-2:	ret
+	ret
 1:	fldl2e
 	fmulp
 	mov $0xc2820000,%eax
diff --git a/src/math/i386/log1p.s b/src/math/i386/log1p.s
index 6b6929c7..354f391a 100644
--- a/src/math/i386/log1p.s
+++ b/src/math/i386/log1p.s
@@ -16,9 +16,6 @@ log1p:
 	fyl2x
 	ret
 		# subnormal x, return x with underflow
-2:	fnstsw %ax
-	and $16,%ax
-	jnz 1f
-	fsts 4(%esp)
+2:	fsts 4(%esp)
 	fstp %st(1)
-1:	ret
+	ret
diff --git a/src/math/i386/log1pf.s b/src/math/i386/log1pf.s
index c0bcd30f..4d3484cd 100644
--- a/src/math/i386/log1pf.s
+++ b/src/math/i386/log1pf.s
@@ -16,10 +16,7 @@ log1pf:
 	fyl2x
 	ret
 		# subnormal x, return x with underflow
-2:	fnstsw %ax
-	and $16,%ax
-	jnz 1f
-	fxch
+2:	fxch
 	fmul %st(1)
 	fstps 4(%esp)
-1:	ret
+	ret

Re: CVE request: musl libc 1.1.23 and earlier x87 float stack imbalance

[Moritz Muehlenhoff]

On Mon, Aug 05, 2019 at 07:27:37PM -0400, Rich Felker wrote:
> I've discovered a flaw in musl libc's arch-specific math assembly code
> for i386, whereby at least the log1p function and possibly others
> return with more than one item on the x87 stack.

Given that the Subject: mentions a CVE request; these are no longer
handled via the oss-security mailing list. Please use
https://cveform.mitre.org/ instead.

Cheers,
        Moritz

Re: CVE request: musl libc 1.1.23 and earlier x87 float stack imbalance

[Rich Felker]

On Mon, Aug 05, 2019 at 08:05:39PM -0400, Rich Felker wrote:
> On Mon, Aug 05, 2019 at 07:27:37PM -0400, Rich Felker wrote:
> > I've discovered a flaw in musl libc's arch-specific math assembly code
> > for i386, whereby at least the log1p function and possibly others
> > return with more than one item on the x87 stack. This can lead to x87
> > stack overflow in the execution of subsequent math code, causing it to
> > incorrectly produce a NAN in place of the actual result. If floating
> > point results are used in flow control, this can lead to runaway wrong
> > code execution. For example, in Python (version 3.6.8 tested), at
> > least one code path of the dtoa function becomes an infinite loop
> > performing what's effectively an unbounded-length memset when entered
> > under such a condition.
> > 
> > This bug is potentially exploitable in software which calls affected
> > math functions with inputs under user control. Impact depends on how
> > the application handles the ABI-violating x87 state; in Python it
> > seems to be limited to producing a crash.
> > 
> > The bug is present in all versions after 0.9.12, up through the
> > current (1.1.23) release. Only 32-bit x86 systems (aka IA32, musl's
> > "i386" arch) are affected. Users of other archs, including x86_64, can
> > safely ignore this issue.
> > 
> > Affected users are advised to apply the following patch:
> > 
> > https://git.musl-libc.org/cgit/musl/patch/?id=f3ed8bfe8a82af1870ddc8696ed4cc1d5aa6b441
> 
> The patch contains an error that was missed for unknown reasons,
> probably failure to rebuild a file. I'm attaching an aggregate patch
> that works. Alternaatively, these two commits can be applied:
> 
> https://git.musl-libc.org/cgit/musl/patch/?id=f3ed8bfe8a82af1870ddc8696ed4cc1d5aa6b441
> https://git.musl-libc.org/cgit/musl/patch/?id=6818c31c9bc4bbad5357f1de14bedf781e5b349e

CVE-2019-14697 has been assigned for this issue.