bpf: Preserve pointer spill metadata during half-slot cleanup

__clean_func_state() cleans dead stack slots in 4-byte halves. When the
high half of a STACK_SPILL slot is dead and the low half remains live,
cleanup converts the live low half to STACK_MISC or STACK_ZERO and clears
the saved spilled_ptr metadata.

That conversion is safe only for scalar spills. For a pointer spill, this
metadata clear lets a later 32-bit fill from the still-live half avoid the
normal non-scalar register-fill check and be treated as an ordinary scalar
stack read.

Leave non-scalar spill slots intact in this half-live shape. This is
conservative for pruning and preserves the existing
check_stack_read_fixed_off() rejection path for partial fills from pointer
spills.

Fixes: be23266b4a08 ("bpf: 4-byte precise clean_verifier_state")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
Link: https://lore.kernel.org/r/20260617-f01-06-half-slot-pointer-spill-v2-1-42b9cdc3cf64@mails.tsinghua.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
index 32f346ce3ffc..ea2153cf28d0 100644
--- a/kernel/bpf/states.c
+++ b/kernel/bpf/states.c
@@ -436,12 +436,10 @@ static void __clean_func_state(struct bpf_verifier_env *env,
 				continue;
 
 			/*
-			 * Only destroy spilled_ptr when hi half is dead.
-			 * If hi half is still live with STACK_SPILL, the
-			 * spilled_ptr metadata is needed for correct state
-			 * comparison in stacksafe().
-			 * is_spilled_reg() is using slot_type[7], but
-			 * is_spilled_scalar_after() check either slot_type[0] or [4]
+			 * Only scalar spills can be degraded to raw stack bytes
+			 * when their high half is dead. Pointer spills need the
+			 * saved spilled_ptr metadata so partial fills keep
+			 * rejecting as non-scalar register fills.
 			 */
 			if (!hi_live) {
 				struct bpf_reg_state *spill = &st->stack[i].spilled_ptr;
@@ -449,6 +447,9 @@ static void __clean_func_state(struct bpf_verifier_env *env,
 				if (lo_live && stype == STACK_SPILL) {
 					u8 val = STACK_MISC;
 
+					if (spill->type != SCALAR_VALUE)
+						continue;
+
 					/*
 					 * 8 byte spill of scalar 0 where half slot is dead
 					 * should become STACK_ZERO in lo 4 bytes.