On Tue, Dec 10, 2024 at 6:52 PM Andrii Nakryiko <[email protected]> wrote: > On Tue, Dec 10, 2024 at 7:34 AM Jann Horn <[email protected]> wrote: > > Currently, the pointer stored in call->prog_array is loaded in > > __uprobe_perf_func(), with no RCU annotation and no immediately visible > > RCU protection, so it looks as if the loaded pointer can immediately be > > dangling. > > Later, bpf_prog_run_array_uprobe() starts a RCU-trace read-side critical > > section, but this is too late. It then uses rcu_dereference_check(), but > > this use of rcu_dereference_check() does not actually dereference anything. > > > > Fix it by aligning the semantics to bpf_prog_run_array(): Let the caller > > provide rcu_read_lock_trace() protection and then load call->prog_array > > with rcu_dereference_check(). > > > > This issue seems to be theoretical: I don't know of any way to reach this > > code without having handle_swbp() further up the stack, which is already > > holding a rcu_read_lock_trace() lock, so where we take > > rcu_read_lock_trace() in __uprobe_perf_func()/bpf_prog_run_array_uprobe() > > doesn't actually have any effect. > > > > Fixes: 8c7dcb84e3b7 ("bpf: implement sleepable uprobes by chaining gps") > > Suggested-by: Andrii Nakryiko <[email protected]> > > Signed-off-by: Jann Horn <[email protected]> > > --- > > Changes in v3: > > - align semantics with bpf_prog_run_array() > > - correct commit message: the issue is theoretical > > - remove stable CC > > - Link to v2: > > https://lore.kernel.org/r/[email protected] > > > > Changes in v2: > > - remove diff chunk in patch notes that confuses git > > - Link to v1: > > https://lore.kernel.org/r/[email protected] > > --- > > include/linux/bpf.h | 11 +++-------- > > kernel/trace/trace_uprobe.c | 6 +++++- > > 2 files changed, 8 insertions(+), 9 deletions(-) > > > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h > > index > > eaee2a819f4c150a34a7b1075584711609682e4c..7fe5cf181511d543b1b100028db94ebb2a44da5d > > 100644 > > --- a/include/linux/bpf.h > > +++ b/include/linux/bpf.h > > @@ -2193,26 +2193,22 @@ bpf_prog_run_array(const struct bpf_prog_array > > *array, > > * rcu-protected dynamically sized maps. > > */ > > static __always_inline u32 > > -bpf_prog_run_array_uprobe(const struct bpf_prog_array __rcu *array_rcu, > > +bpf_prog_run_array_uprobe(const struct bpf_prog_array *array, > > const void *ctx, bpf_prog_run_fn run_prog) > > { > > const struct bpf_prog_array_item *item; > > const struct bpf_prog *prog; > > - const struct bpf_prog_array *array; > > struct bpf_run_ctx *old_run_ctx; > > struct bpf_trace_run_ctx run_ctx; > > u32 ret = 1; > > > > might_fault(); > > + RCU_LOCKDEP_WARN(!rcu_read_lock_trace_held(), "no rcu lock held"); > > > > - rcu_read_lock_trace(); > > migrate_disable(); > > > > run_ctx.is_uprobe = true; > > > > - array = rcu_dereference_check(array_rcu, > > rcu_read_lock_trace_held()); > > - if (unlikely(!array)) > > - goto out; > > I think we should keep this unlikely(NULL) check, bpf_prog_run_array() > has it and see bpf_prog_array_valid() comment below
Whoops, yeah, I removed it here at some point while moving the dereference around and then forgot to re-add it; will fix.
