[PATCH v2 4/5] lib: scatterlist: guard sg_split_phys()/sg_split_mapped() against zero-nents splits

[Charles Pellegrini <c4ffein.work@gmail.com>]

If a caller passes a split_sizes[] array whose trailing entry is 0 and
the preceding splits exactly consume the input scatterlist,
sg_calculate_split() returns success with the trailing split's nents
left at 0. sg_split() then allocates that split's output with

    kmalloc_objs(struct scatterlist, 0, gfp_mask)

which returns ZERO_SIZE_PTR. Both copiers finish each split with a fixup of
the last written output entry -- sg_split_phys() does

    out_sg[-1].length = split->length_last_sg;
    sg_mark_end(out_sg - 1);

and sg_split_mapped() does

    sg_dma_len(--out_sg) = split->length_last_sg;

For a zero-nents split out_sg has not advanced, so that fixup dereferences
ZERO_SIZE_PTR: an out-of-bounds write. KASAN reports it as a slab
out-of-bounds write; a userspace ASAN harness reproduces it as a heap
overflow.

Skip empty splits at the top of the per-split loop in both copiers:

    if (!split->nents)
        continue;

No in-tree caller passes such a split_sizes[] array, so this is latent; it
was surfaced by an automated review of the v1 posting. The fix tolerates
and skips the degenerate split rather than rejecting it with -EINVAL in
sg_calculate_split(); if a stricter contract is preferred, the rejecting
variant is a trivial respin.

Fixes: f8bcbe62acd0 ("lib: scatterlist: add sg splitting function")
Link: https://lore.kernel.org/all/20260601175549.b4a10c07dd9e3b867f66da0c@linux-foundation.org/
Assisted-by: Claude:claude-opus-4-8 hegel-c
Signed-off-by: Charles Pellegrini <c4ffein.work@gmail.com>
---
 lib/sg_split.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/sg_split.c b/lib/sg_split.c
index ab574dd26b03..e90a1bfbdb56 100644
--- a/lib/sg_split.c
+++ b/lib/sg_split.c
@@ -81,6 +81,8 @@ static void sg_split_phys(struct sg_splitter *splitters, const int nb_splits)
 	struct sg_splitter *split;
 
 	for (i = 0, split = splitters; i < nb_splits; i++, split++) {
+		if (!split->nents)
+			continue;
 		in_sg = split->in_sg0;
 		out_sg = split->out_sg;
 		for (j = 0; j < split->nents; j++, out_sg++) {
@@ -108,6 +110,8 @@ static void sg_split_mapped(struct sg_splitter *splitters, const int nb_splits)
 	struct sg_splitter *split;
 
 	for (i = 0, split = splitters; i < nb_splits; i++, split++) {
+		if (!split->nents)
+			continue;
 		in_sg = split->in_sg0;
 		out_sg = split->out_sg;
 		for (j = 0; j < split->nents; j++, out_sg++) {
-- 
2.47.3