CVE-2026-23207: spi: tegra210-quad: Protect curr_xfer check in IRQ handler

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

spi: tegra210-quad: Protect currxfer check in IRQ handler

Now that all other accesses to currxfer are done under the lock, protect the currxfer NULL check in tegraqspiisrthread() with the spinlock. Without this protection, the following race can occur:

CPU0 (ISR thread) CPU1 (timeout path) ---------------- ------------------- if (!tqspi->currxfer) // sees non-NULL spinlock() tqspi->currxfer = NULL spinunlock() handlexfer() spinlock() t = tqspi->currxfer // NULL! ... t->len ... // NULL dereference!

With this patch, all currxfer accesses are now properly synchronized.

Although all accesses to currxfer are done under the lock, in tegraqspiisrthread() it checks for NULL, releases the lock and reacquires it later in handlecpubasedxfer()/handledmabasedxfer(). There is a potential for an update in between, which could cause a NULL pointer dereference.

To handle this, add a NULL check inside the handlers after acquiring the lock. This ensures that if the timeout path has already cleared currxfer, the handler will safely return without dereferencing the NULL pointer.