[PATCH v2 3/3] platform/x86: panasonic-laptop: Fix sentinel write past pcc->sinf[]

[Hilgad Montelo <hilgad.montelo@gmail.com>]

acpi_pcc_retrieve_biosdata() rejects SINF packages only when
pcc->num_sifr is strictly less than hkey->package.count, then
unconditionally writes a trailing sentinel at
pcc->sinf[hkey->package.count]. But pcc->sinf[] is allocated with
exactly pcc->num_sifr elements (valid indices 0..num_sifr-1), so that
write needs num_sifr strictly greater than package.count to stay in
bounds -- num_sifr == package.count passes the existing check but
still overflows by one element.

This is exactly the case probe()'s existing num_sifr++ workaround
("Some DSDT-s have an off-by-one bug where the SINF package count is
one higher than the SQTY reported value") is written to accommodate:
when a DSDT's SINF package count equals SQTY+1, the workaround makes
num_sifr equal to package.count, which is precisely the boundary that
overflows here. Found via UBSan (array-index-out-of-bounds) on
hardware where HKEY.SQTY returns 37 and HKEY.SINF()'s package has 38
elements: num_sifr becomes 38 after the += 1 workaround, the loop
correctly fills indices 0..37, and the sentinel write then targets
index 38, one past the end -- a silent 4-byte heap overflow on kernels
without CONFIG_UBSAN.

Tightening the rejection check to num_sifr <= package.count would
avoid the overflow but breaks probe() entirely on exactly this
hardware, since num_sifr == package.count is the case the off-by-one
workaround exists to support. Nothing else in the driver reads this
sentinel value back, so simply skip the write when there is no room
for it instead.

Signed-off-by: Hilgad Montelo <hilgad.montelo@gmail.com>
---
 drivers/platform/x86/panasonic-laptop.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/platform/x86/panasonic-laptop.c b/drivers/platform/x86/panasonic-laptop.c
index 93e6511..9511440 100644
--- a/drivers/platform/x86/panasonic-laptop.c
+++ b/drivers/platform/x86/panasonic-laptop.c
@@ -476,7 +476,16 @@ static int acpi_pcc_retrieve_biosdata(struct pcc_acpi *pcc)
 		} else
 			pr_err("Invalid HKEY.SINF data\n");
 	}
-	pcc->sinf[hkey->package.count] = -1;
+	/*
+	 * pcc->sinf[] has pcc->num_sifr elements (valid indices
+	 * 0..num_sifr-1). On DSDTs where SINF's package count equals
+	 * num_sifr exactly -- the off-by-one case probe()'s num_sifr++
+	 * already allocates a spare element for -- there is no room left
+	 * for this trailing sentinel; nothing reads it back, so just skip
+	 * the write rather than running one element past the flex array.
+	 */
+	if (hkey->package.count < pcc->num_sifr)
+		pcc->sinf[hkey->package.count] = -1;
 
 end:
 	kfree(buffer.pointer);
-- 
2.53.0