CVE-2026-74276: spi: xilinx: use FIFO occupancy register to determine buffer size

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

spi: xilinx: use FIFO occupancy register to determine buffer size

The method the driver uses to determine the size of the FIFO has a
problem. What it currently does is this:
It stops the SPI hardware and writes to the TX FIFO register until TX
FIFO FULL asserts in the status register. But the hardware does not only
have the FIFO, it also has a shift register which can hold a byte. This
can be seen, when writing a byte to the FIFO (while the SPI hardware is
stopped,) the TX FIFO EMPTY is still empty. So, if we have a FIFO size
of 16 for example, the current method returns a 17.
This is a problem, at least when using the driver in irq mode. The same
size determined for the TX FIFO is also assumed for the RX FIFO. When a
SPI transaction wants to write the amount of the FIFO size or more
bytes, the following happens, for example with 16 bytes FIFO size:
The driver stops the SPI hardware and writes 17 bytes to the TX FIFO and
starts the SPI hardware and goes sleep.
The hardware then shifts out 17 bytes (FIFO + shift register) and
simultaneously reads bytes into the RX FIFO, but it only has 16 places,
so it looses one byte. Then TX FIFO empty asserts, wakes the driver
again, which has a fast path and reads 16 bytes from the RX FIFO, but
before reading the last 17th byte (which is lost) it does this:

	sr = xspi->read_fn(xspi->regs + XSPI_SR_OFFSET);
	if (!(sr & XSPI_SR_RX_EMPTY_MASK)) {
		xilinx_spi_rx(xspi);
		rx_words--;
	}

It reads the status register and checks if the RX FIFO is not empty.
But it is empty in our case. So this check spins in a while loop
forever locking the driver.

This patch fixes the logic to determine the FIFO size.

The Linux kernel CVE team has assigned CVE-2026-74276 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 5.10.261 with commit 4ee65558a7fda463222f5edc631b6d7862218725
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 5.15.212 with commit e98f589d54068dfcc7da42d9b730e47b84e2991a
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 6.1.178 with commit 1fc6a6c0cc7796b0c1cf4eec3f0720f422f09273
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 6.6.145 with commit dfab0d4698a642bab9b895e2e6d240838cfe2cc6
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 6.12.97 with commit d7ccd8736b67550eca746ed7fa39a36016ff114b
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 6.18.40 with commit 1c9246a199e19b2fd36e94feed60e55f27103a4f
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 7.1.5 with commit d1944b71c18e7494756bff1a6f80c25be99eaecb
	Issue introduced in 4.0 with commit 4c9a761402d780b86b9b068aba4ef8e29ed15e99 and fixed in 7.2-rc1 with commit 47f3b5365536e8c38f264824ab15fdb74454e066

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74276
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/spi/spi-xilinx.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4ee65558a7fda463222f5edc631b6d7862218725
	https://git.kernel.org/stable/c/e98f589d54068dfcc7da42d9b730e47b84e2991a
	https://git.kernel.org/stable/c/1fc6a6c0cc7796b0c1cf4eec3f0720f422f09273
	https://git.kernel.org/stable/c/dfab0d4698a642bab9b895e2e6d240838cfe2cc6
	https://git.kernel.org/stable/c/d7ccd8736b67550eca746ed7fa39a36016ff114b
	https://git.kernel.org/stable/c/1c9246a199e19b2fd36e94feed60e55f27103a4f
	https://git.kernel.org/stable/c/d1944b71c18e7494756bff1a6f80c25be99eaecb
	https://git.kernel.org/stable/c/47f3b5365536e8c38f264824ab15fdb74454e066