CVE-2023-52499

In the Linux kernel, the following vulnerability has been resolved:
powerpc/47x: Fix 47x syscall return crash Eddie reported that newer kernels
were crashing during boot on his 476 FSP2 system: kernel tried to execute
user page (b7ee2000) - exploit attempt? (uid: 0) BUG: Unable to handle
kernel instruction fetch Faulting instruction address: 0xb7ee2000 Oops:
Kernel access of bad area, sig: 11 [#1] BE PAGE_SIZE=4K FSP-2 Modules
linked in: CPU: 0 PID: 61 Comm: mount Not tainted 6.1.55-d23900f.ppcnf-fsp2
#1 Hardware name: ibm,fsp2 476fpe 0x7ff520c0 FSP-2 NIP: b7ee2000 LR:
8c008000 CTR: 00000000 REGS: bffebd83 TRAP: 0400 Not tainted
(6.1.55-d23900f.ppcnf-fs p2) MSR: 00000030 <IR,DR> CR: 00001000 XER:
20000000 GPR00: c00110ac bffebe63 bffebe7e bffebe88 8c008000 00001000
00000d12 b7ee2000 GPR08: 00000033 00000000 00000000 c139df10 48224824
1016c314 10160000 00000000 GPR16: 10160000 10160000 00000008 00000000
10160000 00000000 10160000 1017f5b0 GPR24: 1017fa50 1017f4f0 1017fa50
1017f740 1017f630 00000000 00000000 1017f4f0 NIP [b7ee2000] 0xb7ee2000 LR
[8c008000] 0x8c008000 Call Trace: Instruction dump: XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX —[ end trace
0000000000000000 ]— The problem is in ret_from_syscall where the check
for icache_44x_need_flush is done. When the flush is needed the code jumps
out-of-line to do the flush, and then intends to jump back to continue the
syscall return. However the branch back to label 1b doesn’t return to the
correct location, instead branching back just prior to the return to
userspace, causing bogus register values to be used by the rfi. The
breakage was introduced by commit 6f76a01173cc (“powerpc/syscall: implement
system call entry/exit logic in C for PPC32”) which inadvertently removed
the “1” label and reused it elsewhere. Fix it by adding named local labels
in the correct locations. Note that the return label needs to be outside
the ifdef so that CONFIG_PPC_47x=n compiles.