Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [Medium] dyndbg: race during module load bypasses ABI protection permanently --
commit c8a052c030fff7b63aa2d9e4c2dc3aa455948704 Author: Jim Cromie <[email protected]> dyndbg: resolve "protection" of class'd pr_debug This patch implements an ABI protection mechanism for dynamically debugged classes by binding a controlling sysfs parameter to classmaps. If a module parameter is configured, it protects class'd callsites from being accidentally modified by un-classed wildcard queries. > diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c > index 636a88994a774..d300d44c1a82e 100644 > --- a/lib/dynamic_debug.c > +++ b/lib/dynamic_debug.c [ ... ] > @@ -252,7 +294,28 @@ static bool ddebug_match_desc(const struct ddebug_query > *query, > dp->lineno > query->last_lineno) > return false; > > - return true; > + /* > + * above are all satisfied, so we can make final decisions: > + * 1- class FOO or implied class __DEFAULT__ > + * 2- site.is_classed or not > + */ > + if (query->class_string) { > + /* class FOO given, exact match required */ > + return (dp->class_id == selected_class); > + } > + /* query class __DEFAULT__ by omission. */ > + if (dp->class_id == _DPRINTK_CLASS_DFLT) { > + /* un-classed site */ > + return true; > + } > + /* site is class'd */ > + site_map = ddebug_find_map_by_class_id(di, dp->class_id); > + if (!site_map) { > + WARN_ONCE(1, "unknown class_id %d, check %s's CLASSMAP > definitions", dp->class_id, di->mod_name); > + return false; > + } > + /* module(-param) decides protection */ > + return !ddebug_class_wants_protection(site_map); > } [Severity: Medium] Does this code expose a race condition when a module is loading? Looking at ddebug_add_module(), the module is added to ddebug_tables and the ddebug_lock is dropped before map->controlling_param is initialized: lib/dynamic_debug.c:ddebug_add_module() { ... mutex_lock(&ddebug_lock); list_add_tail(&dt->link, &ddebug_tables); mutex_unlock(&ddebug_lock); if (dt->info.maps.len) ddebug_apply_class_maps(&dt->info); ... } If a concurrent wildcard query (like echo -p > /proc/dynamic_debug/control) is processed while the lock is dropped but before ddebug_apply_class_maps() finishes binding the parameter, site_map->controlling_param will be NULL. This causes ddebug_class_wants_protection() to return false, bypassing the protection and erroneously enabling the callsites. [ ... ] > @@ -772,11 +833,14 @@ static int param_set_dyndbg_module_classes(const char > *instr, > const struct kernel_param *kp, > const char *mod_name) > { > - const struct ddebug_class_param *dcp = kp->arg; > - const struct ddebug_class_map *map = dcp->map; > + struct ddebug_class_param *dcp = kp->arg; > + struct ddebug_class_map *map = dcp->map; > u32 inrep, new_bits, old_bits, old_val; > int rc, totct = 0; > > + if (map && !map->controlling_param) > + map->controlling_param = dcp; [Severity: Medium] Is it safe to assign map->controlling_param here without holding ddebug_lock or using WRITE_ONCE()? Since this shared state is accessed by concurrent queries, could this cause a data race? [ ... ] > @@ -1332,25 +1395,40 @@ static void ddebug_sync_classbits(const struct > kernel_param *kp, const char *mod > } > } > > -static void ddebug_match_apply_kparam(const struct kernel_param *kp, > - const struct ddebug_class_map *map, > - const char *mod_name) > +static struct ddebug_class_param * > +ddebug_get_classmap_kparam(const struct kernel_param *kp, > + const struct ddebug_class_map *map) > +{ > + struct ddebug_class_param *dcp; > + > + if (kp->ops != ¶m_ops_dyndbg_classes) > + return NULL; > + > + dcp = (struct ddebug_class_param *)kp->arg; > + return (map == dcp->map) > + ? dcp : (struct ddebug_class_param *)NULL; > +} > + > +static void ddebug_match_apply_kparam(const struct kernel_param *kp, > + struct ddebug_class_map *map, > + const char *mod_name) > { > + struct ddebug_class_param *dcp = ddebug_get_classmap_kparam(kp, map); > > if (dcp && dcp->map == map) { > + /* > + * Bind controlling_param to activate ABI protection in > + * ddebug_match_desc(), shielding callsites from non-class > + * wildcard (>control) queries. > + */ > + map->controlling_param = dcp; > v2pr_info(" kp:%s.%s =0x%x", mod_name, kp->name, *dcp->bits); > vpr_cm_info(map, " %s maps ", mod_name); > ddebug_sync_classbits(kp, mod_name); > } > } [Severity: Medium] Can the delayed assignment of map->controlling_param combined with ddebug_sync_classbits() result in permanent desynchronization? During the race window in ddebug_add_module() mentioned above, callsites can be erroneously enabled by a concurrent query. Once the module loading thread resumes, it binds map->controlling_param here and calls ddebug_sync_classbits(). However, ddebug_sync_classbits() passes 0 for old_bits: lib/dynamic_debug.c:ddebug_sync_classbits() { ... case DD_CLASS_TYPE_DISJOINT_BITS: val = READ_ONCE(*dcp->bits); new_bits = val; v2pr_info(" %s: classbits: 0x%x\n", KP_NAME(kp), new_bits); ddebug_apply_class_bitmap(dcp, &new_bits, 0, modname); ... } Because old_bits is 0, the function skips resetting any class bits that are supposed to be off (since it sees new_bits (0) == old_bits (0)). Doesn't this prevent the synchronizer from noticing and clearing the erroneously enabled callsites? -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=32
