Re: CVE-2024-35918: randomize_kstack: Improve entropy diffusion

[Michal Koutný]

Hello.

On Sun, May 19, 2024 at 12:11:12PM GMT, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> randomize_kstack: Improve entropy diffusion
> 
> The kstack_offset variable was really only ever using the low bits for
> kernel stack offset entropy. Add a ror32() to increase bit diffusion.
> 
> The Linux kernel CVE team has assigned CVE-2024-35918 to this issue.
> 
> 
> Affected and fixed versions
> ===========================
> 
> 	Issue introduced in 5.13 with commit 39218ff4c625 and fixed in 5.15.155 with commit dfb2ce952143
> 	Issue introduced in 5.13 with commit 39218ff4c625 and fixed in 6.1.86 with commit e80b4980af26
> 	Issue introduced in 5.13 with commit 39218ff4c625 and fixed in 6.6.27 with commit 300a2b9c2b28
> 	Issue introduced in 5.13 with commit 39218ff4c625 and fixed in 6.8.6 with commit 6be74b1e21f8
> 	Issue introduced in 5.13 with commit 39218ff4c625 and fixed in 6.9 with commit 9c573cd31343

The commit
9c573cd313433 ("randomize_kstack: Improve entropy diffusion") v6.9-rc4~35^2
adds ~2 bits of entropy to stack offsets (+the diffusion, x86_64)

The commit
39218ff4c625d ("stack: Optionally randomize kernel stack offset each syscall") v5.13-rc1~184^2~3
adds ~8 bit of entropy to stack offsets (there was none before, x86_64)

Why the former commit has a CVE while the latter doesn't? (2 < 8)

I'd expect both to be treated equally or even inversely.

Thanks,
Michal

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