CVE-2025-37991: parisc: Fix double SIGFPE crash

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

parisc: Fix double SIGFPE crash

Camm noticed that on parisc a SIGFPE exception will crash an application with a second SIGFPE in the signal handler. Dave analyzed it, and it happens because glibc uses a double-word floating-point store to atomically update function descriptors. As a result of lazy binding, we hit a floating-point store in fpefunc almost immediately.

When the T bit is set, an assist exception trap occurs when when the co-processor encounters any floating-point instruction except for a double store of register %fr0. The latter cancels all pending traps. Let's fix this by clearing the Trap (T) bit in the FP status register before returning to the signal handler in userspace.

The issue can be reproduced with this test program:

root@parisc:~# cat fpe.c

static void fpefunc(int sig, siginfot i, void v) { sigsett set; sigemptyset(&set); sigaddset(&set, SIGFPE); sigprocmask(SIGUNBLOCK, &set, NULL); printf("GOT signal %d with sicode %ld\n", sig, i->sicode); }

int main() { struct sigaction action = { .sasigaction = fpefunc, .saflags = SARESTART|SASIGINFO }; sigaction(SIGFPE, &action, 0); feenableexcept(FEOVERFLOW); return printf("%lf\n",1.7976931348623158E3081.7976931348623158E308); }

root@parisc:~# gcc fpe.c -lm root@parisc:~# ./a.out Floating point exception

root@parisc:~# strace -f ./a.out execve("./a.out", ["./a.out"], 0xf9ac7034 / 20 vars /) = 0 getrlimit(RLIMITSTACK, {rlimcur=81921024, rlimmax=RLIMINFINITY}) = 0 ... rtsigaction(SIGFPE, {sahandler=0x1110a, samask=[], saflags=SARESTART|SASIGINFO}, NULL, 8) = 0 --- SIGFPE {sisigno=SIGFPE, sicode=FPEFLTOVF, siaddr=0x1078f} --- --- SIGFPE {sisigno=SIGFPE, sicode=FPEFLTOVF, siaddr=0xf8f21237} --- +++ killed by SIGFPE +++ Floating point exception