Re: [PATCH 6.12.y v2] mm/damon/core: fix unconditionally skip last region

[SJ Park <sj@kernel.org>]

On Wed, 30 Sep 2026 10:26:49 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] mm: OOB dereference in damon_do_apply_schemes() on split failure
> --
> 
> commit 61e0ff2e7385bb544355d1f3aa3598f7f8e9e1d0
> Author: Liew Rui Yan <aethernet65535@gmail.com>
> 
> mm/damon/core: fix unconditionally skip last region
> 
> This commit changes the logic in damos_skip_charged_region() to only reset
> charge_{target,addr}_from when the last region is reached. This is intended to
> ensure the last region is not skipped if it has not been fully processed.
> 
> Link: https://lore.kernel.org/20260908134739.96919-1-sj@kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 32737cc5f877b..cc7250f0936af 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> [ ... ]
> > @@ -1322,37 +1322,41 @@ static bool damos_skip_charged_region(struct damon_target *t,
> >  	struct damon_region *r = *rp;
> >  	struct damos_quota *quota = &s->quota;
> >  	unsigned long sz_to_skip;
> > +	bool skip = false;
> >  
> >  	/* Skip previously charged regions */
> >  	if (quota->charge_target_from) {
> >  		if (t != quota->charge_target_from)
> >  			return true;
> > -		if (r == damon_last_region(t)) {
> > -			quota->charge_target_from = NULL;
> > -			quota->charge_addr_from = 0;
> > -			return true;
> > -		}
> >  		if (quota->charge_addr_from &&
> > -				r->ar.end <= quota->charge_addr_from)
> > -			return true;
> > +				r->ar.end <= quota->charge_addr_from) {
> > +			skip = true;
> > +			goto out;
> > +		}
> >  
> >  		if (quota->charge_addr_from && r->ar.start <
> >  				quota->charge_addr_from) {
> >  			sz_to_skip = ALIGN_DOWN(quota->charge_addr_from -
> >  					r->ar.start, DAMON_MIN_REGION);
> >  			if (!sz_to_skip) {
> > -				if (damon_sz_region(r) <= DAMON_MIN_REGION)
> > -					return true;
> > +				if (damon_sz_region(r) <= DAMON_MIN_REGION) {
> > +					skip = true;
> > +					goto out;
> > +				}
> >  				sz_to_skip = DAMON_MIN_REGION;
> >  			}
> >  			damon_split_region_at(t, r, sz_to_skip);
> >  			r = damon_next_region(r);
> >  			*rp = r;
> > +			skip = false;
> >  		}
> > +	}
> > +out:
> > +	if (r == damon_last_region(t)) {
> 
> [Severity: High]
> If damon_split_region_at() fails to allocate memory, it returns silently and
> the region remains unmodified. If r happens to be the last region in the list,
> could damon_next_region(r) evaluate past the list boundary and return an
> invalid pointer?
> 
> Since this invalid pointer is assigned to *rp and the function continues to
> the out label, could it fail this (r == damon_last_region(t)) check, bypass
> the intended quota reset, and return false?
> 
> If so, could the caller damon_do_apply_schemes() then receive this invalid
> pointer and pass it to damos_valid_target():
> 
> mm/damon/core.c:damon_do_apply_schemes() {
>     ...
> 		if (damos_skip_charged_region(t, &r, s))
> 			continue;
> 
> 		if (!damos_valid_target(c, t, r, s))
> 			continue;
>     ...
> }
> 
> causing it to dereference garbage data (such as r->ar.start and r->ar.end) and
> potentially lead to an out-of-bounds memory access?

I will separately work on this.  Refer to my other reply to same issue report
for details.

[1] https://lore.kernel.org/20260930102333.58847-1-sj@kernel.org

> 
> >  		quota->charge_target_from = NULL;
> >  		quota->charge_addr_from = 0;
> >  	}
> > -	return false;
> > +	return skip;
> >  }
> >  
> >  static void damos_update_stat(struct damos *s,
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260930101459.56041-1-sj@kernel.org?part=1


Thanks,
SJ