Re: [PATCH] tracing/histograms: Reject non-stacktrace fields for synthetic stacktrace fields

[Lee Jones <lee@kernel.org>]

On Mon, 12 Oct 2026, sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [High] tracing: check_synth_field regression for __bitmask and stacktraces
> - [High] tracing: arbitrary memory read via scalar to string synthetic field mapping

Fixing now - v2 to follow shortly.  Bear with.

> --
> 
> commit 4813d717e995df13871068dd6fd996711ca8965d
> Author: Lee Jones <lee@kernel.org>
> 
> tracing/histograms: Reject non-stacktrace fields for synthetic stacktrace fields
> 
> In check_synth_field(), when a synthetic event field is a stacktrace
> and the histogram field is not a "long[" stacktrace array, validation falls
> through to the generic size and signedness comparison.  This allows any
> 8-byte signed scalar histogram field to pass the fallback check and be
> dereferenced as a kernel stacktrace pointer.  This commit attempts to reject
> these mismatches by enforcing that field->is_stack matches the "long[" type.
> 
> > diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> > index 8af97fd4ee2d5..b09f5043e1543 100644
> > --- a/kernel/trace/trace_events_hist.c
> > +++ b/kernel/trace/trace_events_hist.c
> > @@ -3946,7 +3946,10 @@ static int check_synth_field(struct synth_event *event,
> >  	    && field->is_dynamic)
> >  		return 0;
> >  
> > -	if (strstr(hist_field->type, "long[") && field->is_stack)
> > +	if (!!strstr(hist_field->type, "long[") != field->is_stack)
> > +		return -EINVAL;
> 
> [Severity: High]
> Does this heuristic falsely match the long[ substring in __data_loc unsigned
> long[] from __bitmask and __cpumask fields?
> 
> If is_stack == false, this logic seems to falsely reject valid mappings.
> 
> If is_stack == true, this appears to bypass validation and allow a 32-bit
> __data_loc integer to be passed as the field value. Could this 32-bit integer
> then be cast to a pointer and dereferenced in
> trace_events_synth.c:get_field_size() and trace_stack(), potentially causing
> a kernel panic?
> 
> > +
> > +	if (field->is_stack)
> >  		return 0;
> >  
> >  	if (strcmp(field->type, hist_field->type) != 0) {
> >  		if (field->size != hist_field->size ||
> >  		    (!field->is_string && field->is_signed != hist_field->is_signed))
> >  			return -EINVAL;
> >  	}
> 
> [Severity: High]
> It looks like this fallback logic fails to reject a scalar-to-string mapping.
> 
> When creating a dynamic string synthetic field, its size is set to sizeof(u64)
> and is_string is true.  Because field->size equals hist_field->size (8 bytes)
> and !field->is_string short-circuits the type/signedness check, will an 8-byte
> scalar successfully map to a dynamic string?
> 
> If an attacker-controlled scalar is passed, can it be cast to a kernel pointer
> in trace_events_synth.c:trace_string() and used with fetch_store_strlen() to
> arbitrarily read kernel memory?
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261012155805.1103972-1-lee@kernel.org?part=1

-- 
Lee Jones