From dc76258d0132df1d831a5a29758bd448ca9c566e Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Krzysztof=20Wilczy=C5=84ski?= Date: Tue, 16 Jun 2026 16:31:30 +0000 Subject: [PATCH] PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit pci_write_legacy_io() loads 4 bytes from the kernfs write buffer regardless of how many bytes userspace wrote: if (count != 1 && count != 2 && count != 4) return -EINVAL; return pci_legacy_write(bus, off, *(u32 *)buf, count); kernfs_fop_write_iter() allocates the buffer with kmalloc(len + 1), so a 1-byte write to the legacy_io sysfs file allocates 2 bytes and the unconditional u32 load reads up to 2 bytes past the end of the allocation, which KASAN reports as a slab-out-of-bounds read. Similarly, a 2-byte write overreads by 1 byte. Thus, read only the number of bytes requested using get_unaligned_le16() and get_unaligned_le32() for the 2 and 4 byte cases, interpreting the buffer as little-endian to match the byte ordering of PCI I/O port space. The PowerPC implementation previously compensated for the generic code's native-endian 32-bit load by shifting the value into place for the 1 and 2 byte cases. The shifts were only correct on big-endian kernels. On little-endian PowerPC (POWER8 and later), they extracted the wrong bytes, so a 1-byte write wrote an out-of-bounds byte instead of the requested value. On big-endian, the native load also caused out_le16() and out_le32() to reverse the user's bytes on the wire for 2 and 4 byte writes. The little-endian helpers resolve both issues, so the shifts are removed. No changes are needed for the Alpha platform. The legacy_io file is root-only and exists only on Alpha and PowerPC, the two architectures that define HAVE_PCI_LEGACY. Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260616163131.2763281-1-kwilczynski@kernel.org Signed-off-by: Krzysztof Wilczyński --- arch/powerpc/kernel/pci-common.c | 9 ++------- drivers/pci/pci-sysfs.c | 18 +++++++++++++++--- 2 files changed, 17 insertions(+), 10 deletions(-) diff --git a/arch/powerpc/kernel/pci-common.c b/arch/powerpc/kernel/pci-common.c index 3c4ca90e2ab7..4fc52c21fe5d 100644 --- a/arch/powerpc/kernel/pci-common.c +++ b/arch/powerpc/kernel/pci-common.c @@ -626,19 +626,14 @@ int pci_legacy_write(struct pci_bus *bus, loff_t port, u32 val, size_t size) return -ENXIO; addr = hose->io_base_virt + port; - /* WARNING: The generic code is idiotic. It gets passed a pointer - * to what can be a 1, 2 or 4 byte quantity and always reads that - * as a u32, which means that we have to correct the location of - * the data read within those 32 bits for size 1 and 2 - */ switch(size) { case 1: - out_8(addr, val >> 24); + out_8(addr, val); return 1; case 2: if (port & 1) return -EINVAL; - out_le16(addr, val >> 16); + out_le16(addr, val); return 2; case 4: if (port & 3) diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c index 5ec0b245a69b..2970ad502b78 100644 --- a/drivers/pci/pci-sysfs.c +++ b/drivers/pci/pci-sysfs.c @@ -913,12 +913,24 @@ static ssize_t pci_write_legacy_io(struct file *filp, struct kobject *kobj, char *buf, loff_t off, size_t count) { struct pci_bus *bus = to_pci_bus(kobj_to_dev(kobj)); + u32 val; - /* Only support 1, 2 or 4 byte accesses */ - if (count != 1 && count != 2 && count != 4) + /* Only support 1, 2 or 4 byte accesses. */ + switch (count) { + case 1: + val = *(u8 *)buf; + break; + case 2: + val = get_unaligned_le16(buf); + break; + case 4: + val = get_unaligned_le32(buf); + break; + default: return -EINVAL; + } - return pci_legacy_write(bus, off, *(u32 *)buf, count); + return pci_legacy_write(bus, off, val, count); } /**