Messages in this thread | | | Date | Mon, 9 Mar 2020 11:11:54 -0700 | From | Eric Biggers <> | Subject | Re: KMSAN: uninit-value in snapshot_compat_ioctl |
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On Mon, Mar 09, 2020 at 12:53:28PM +0100, 'Alexander Potapenko' via syzkaller-bugs wrote: > > > Looks like a KMSAN false positive? As far as I can tell, the memory is being > > > initialized by put_user() called under set_fs(KERNEL_DS). > > Why? put_user() doesn't write to kernel memory, instead it copies a > value to the userspace. > That's why KMSAN performs kmsan_check_memory() on it. > It would actually be better if KMSAN printed an kernel-infoleak warning instead.
When under set_fs(KERNEL_DS), the userspace access functions like put_user() and copy_to_user() can write to kernel memory. It's discouraged and people have been trying to get rid of uses of set_fs(), but a lot still remain, since sometimes it's useful to allow code to operate on both user and kernel memory. A common example is kernel_read().
> > > Although, it also looks like the problematic code can just be removed, since > > always sizeof(compat_loff_t) == sizeof(loff_t). I'll send a patch to do that... > > Thanks! >
- Eric
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