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

[Rich Felker <dalias@libc.org>]

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.