A malicious signal handler / restorer can DOS the system by fudging the
user regs saved on stack, causing weird things such as sigreturn returning
to user mode PC but cpu state still being kernel mode....
Ensure that in sigreturn path status32 always has U bit; any other bogosity
(gargbage PC etc) will be taken care of by normal user mode exceptions mechanisms.
Reproducer signal handler:
void handle_sig(int signo, siginfo_t *info, void *context)
{
ucontext_t *uc = context;
struct user_regs_struct *regs = &(uc->uc_mcontext.regs);
regs->scratch.status32 = 0;
}
Before the fix, kernel would go off to weeds like below:
--------->8-----------
[ARCLinux]$ ./signal-test
Path: /signal-test
CPU: 0 PID: 61 Comm: signal-test Not tainted 4.0.0-rc5+ #65
task: 8f177880 ti: 5ffe6000 task.ti: 8f15c000
[ECR ]: 0x00220200 => Invalid Write @ 0x00000010 by insn @ 0x00010698
[EFA ]: 0x00000010
[BLINK ]: 0x2007c1ee
[ERET ]: 0x10698
[STAT32]: 0x00000000 : <--------
BTA: 0x00010680 SP: 0x5ffe7e48 FP: 0x00000000
LPS: 0x20003c6c LPE: 0x20003c70 LPC: 0x00000000
...
--------->8-----------
Reported-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
e4140819da