[PATCH v2 1/5] lib: scatterlist: fix sg_calculate_split() nb_splits overshoot on partial coverage

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

sg_calculate_split() decrements nb_splits in two places per outer-loop
iteration: once in the inner while() that advances to the next output
split when a split boundary falls mid-entry, and once in the trailing

    if (!size && --nb_splits > 0)

The loop-termination check only tests for nb_splits == 0:

    if (!nb_splits)
        break;

When the final requested split ends mid-entry of a non-last input entry,
both decrements run with their bodies skipped: the inner while() takes
nb_splits 1 -> 0, then the trailing if() takes it 0 -> -1. The
termination check then sees -1, not 0, so the loop does not break and the
next input entry is folded into the last split, inflating its nents and
length_last_sg. The resulting split covers more bytes than requested.
For in=[2, 1], skip=0, nb_splits=1, sizes=[1] the single output split
reports a total length of 2 instead of 1.

Widen the termination check to also catch the overshoot to -1:

    if (nb_splits <= 0)
        break;

The new condition only fires when nb_splits is already negative, which is
reachable solely through the double decrement above, so it cannot change
behaviour for any input that splits correctly today.

This was first reported by Alexander Egorenkov in 2021, with an identical
root-cause analysis; that patch was never reviewed or merged. Its proposed
fix moved the existing termination check above the second decrement rather
than widening it. That handles the mid-entry case but regresses
edge-aligned partial coverage -- a split ending exactly on an input-entry
boundary with further entries trailing -- which the current code handles
correctly. A KUnit test added later in this series
(sg_split_t10_edge_first_two_trailing) covers that regression.

No in-tree caller triggers the bug today: four request full coverage or
re-split over a DMA-mapped subset, and the one caller that requests
partial coverage (the DTHEv2 AEAD driver) bounds its consumer by cryptlen,
masking the wrong geometry. The fix matters for callers that legitimately
request partial coverage, which the API has documented as supported since
the original commit ("the union of spans of all resulting scatter lists is
a subrange of the span of the original scatter list").

Fixes: f8bcbe62acd0 ("lib: scatterlist: add sg splitting function")
Reported-by: Alexander Egorenkov <egorenar-dev@posteo.net>
Closes: https://lore.kernel.org/all/20210418143425.22944-1-egorenar-dev@posteo.net/
Assisted-by: Claude:claude-opus-4-8 hegel-c
Signed-off-by: Charles Pellegrini <c4ffein.work@gmail.com>
---
 lib/sg_split.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/sg_split.c b/lib/sg_split.c
index 24e8f5e48e63..28fe7fa6c9ac 100644
--- a/lib/sg_split.c
+++ b/lib/sg_split.c
@@ -67,7 +67,7 @@ static int sg_calculate_split(struct scatterlist *in, int nents, int nb_splits,
 			size = *(++sizes);
 		}
 
-		if (!nb_splits)
+		if (nb_splits <= 0)
 			break;
 	}
 
-- 
2.47.3