FAILED: patch "[PATCH] powerpc/47x: Fix 47x syscall return crash" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x f0eee815babed70a749d2496a7678be5b45b4c14
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023101544-licking-mutation-b95d@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f0eee815babed70a749d2496a7678be5b45b4c14 Mon Sep 17 00:00:00 2001
From: Michael Ellerman <[email protected]>
Date: Tue, 10 Oct 2023 22:47:50 +1100
Subject: [PATCH] powerpc/47x: Fix 47x syscall return crash
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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.

index 9692acb0361f..7eda33a24bb4 100644
@@ -137,8 +137,9 @@ ret_from_syscall:
 	lis	r4,icache_44x_need_flush@ha
 	lwz	r5,icache_44x_need_flush@l(r4)
 	cmplwi	cr0,r5,0
-	bne-	2f
+	bne-	.L44x_icache_flush
 #endif /* CONFIG_PPC_47x */
+.L44x_icache_flush_return:
 	kuep_unlock
 	lwz	r4,_LINK(r1)
 	lwz	r5,_CCR(r1)

@@ -172,10 +173,11 @@ syscall_exit_finish:
 	b	1b
 
 #ifdef CONFIG_44x
-2:	li	r7,0
+.L44x_icache_flush:
+	li	r7,0
 	iccci	r0,r0
 	stw	r7,icache_44x_need_flush@l(r4)
-	b	1b
+	b	.L44x_icache_flush_return
 #endif  /* CONFIG_44x */
 
 	.globl	ret_from_fork