Re: CVE-2021-47326: x86/signal: Detect and prevent an alternate signal stack overflow

Re: CVE-2021-47326: x86/signal: Detect and prevent an alternate signal stack overflow

[Nikolay Borisov]

On 21.05.24 г. 17:36 ч., Greg Kroah-Hartman wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> x86/signal: Detect and prevent an alternate signal stack overflow
> 
> The kernel pushes context on to the userspace stack to prepare for the
> user's signal handler. When the user has supplied an alternate signal
> stack, via sigaltstack(2), it is easy for the kernel to verify that the
> stack size is sufficient for the current hardware context.
> 
> Check if writing the hardware context to the alternate stack will exceed
> it's size. If yes, then instead of corrupting user-data and proceeding with
> the original signal handler, an immediate SIGSEGV signal is delivered.
> 
> Refactor the stack pointer check code from on_sig_stack() and use the new
> helper.
> 
> While the kernel allows new source code to discover and use a sufficient
> alternate signal stack size, this check is still necessary to protect
> binaries with insufficient alternate signal stack size from data
> corruption.
> 
> The Linux kernel CVE team has assigned CVE-2021-47326 to this issue.


I'd like to dispute this CVE. Basically a process can pass in a 
wrongly-sized stack which will cause its own stack to be corrupted. If 
anything this affects the process rather than the kernel.

> 
> 
> Affected and fixed versions
> ===========================
> 
> 	Issue introduced in 3.15 with commit c2bc11f10a39 and fixed in 5.4.134 with commit 00fcd8f33e9b
> 	Issue introduced in 3.15 with commit c2bc11f10a39 and fixed in 5.10.52 with commit 74569cb9ed7b
> 	Issue introduced in 3.15 with commit c2bc11f10a39 and fixed in 5.12.19 with commit 74d6fcea1d89
> 	Issue introduced in 3.15 with commit c2bc11f10a39 and fixed in 5.13.4 with commit afb04d0b5543
> 	Issue introduced in 3.15 with commit c2bc11f10a39 and fixed in 5.14 with commit 2beb4a53fc3f
> 	Issue introduced in 3.14.3 with commit 5bc3ab30db9b
> 
> Please see https://www.kernel.org for a full list of currently supported
> kernel versions by the kernel community.
> 
> Unaffected versions might change over time as fixes are backported to
> older supported kernel versions.  The official CVE entry at
> 	https://cve.org/CVERecord/?id=CVE-2021-47326
> will be updated if fixes are backported, please check that for the most
> up to date information about this issue.
> 
> 
> Affected files
> ==============
> 
> The file(s) affected by this issue are:
> 	arch/x86/kernel/signal.c
> 	include/linux/sched/signal.h
> 
> 
> Mitigation
> ==========
> 
> The Linux kernel CVE team recommends that you update to the latest
> stable kernel version for this, and many other bugfixes.  Individual
> changes are never tested alone, but rather are part of a larger kernel
> release.  Cherry-picking individual commits is not recommended or
> supported by the Linux kernel community at all.  If however, updating to
> the latest release is impossible, the individual changes to resolve this
> issue can be found at these commits:
> 	https://git.kernel.org/stable/c/00fcd8f33e9b9f57115c3b1cfc4cb96450c18796
> 	https://git.kernel.org/stable/c/74569cb9ed7bc60e395927f55d3dc3be143a0164
> 	https://git.kernel.org/stable/c/74d6fcea1d896800e60f1c675137efebd1a6c9a6
> 	https://git.kernel.org/stable/c/afb04d0b5543a5bf8e157b9119fbfc52606f4c11
> 	https://git.kernel.org/stable/c/2beb4a53fc3f1081cedc1c1a198c7f56cc4fc60c

Re: CVE-2021-47326: x86/signal: Detect and prevent an alternate signal stack overflow

[Greg Kroah-Hartman]

On Thu, May 23, 2024 at 01:19:17PM +0300, Nikolay Borisov wrote:
> 
> 
> On 21.05.24 г. 17:36 ч., Greg Kroah-Hartman wrote:
> > Description
> > ===========
> > 
> > In the Linux kernel, the following vulnerability has been resolved:
> > 
> > x86/signal: Detect and prevent an alternate signal stack overflow
> > 
> > The kernel pushes context on to the userspace stack to prepare for the
> > user's signal handler. When the user has supplied an alternate signal
> > stack, via sigaltstack(2), it is easy for the kernel to verify that the
> > stack size is sufficient for the current hardware context.
> > 
> > Check if writing the hardware context to the alternate stack will exceed
> > it's size. If yes, then instead of corrupting user-data and proceeding with
> > the original signal handler, an immediate SIGSEGV signal is delivered.
> > 
> > Refactor the stack pointer check code from on_sig_stack() and use the new
> > helper.
> > 
> > While the kernel allows new source code to discover and use a sufficient
> > alternate signal stack size, this check is still necessary to protect
> > binaries with insufficient alternate signal stack size from data
> > corruption.
> > 
> > The Linux kernel CVE team has assigned CVE-2021-47326 to this issue.
> 
> 
> I'd like to dispute this CVE. Basically a process can pass in a
> wrongly-sized stack which will cause its own stack to be corrupted. If
> anything this affects the process rather than the kernel.

Ah, good catch, thanks, I'll go revoke this CVE now.

Thanks for the review, much appreciated!

greg k-h