The patch below does not apply to the 5.10-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 stable@vger.kernel.org.

thanks,

greg k-h

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

From 24321ac668e452a4942598533d267805f291fdc9 Mon Sep 17 00:00:00 2001

From: Raoni Fassina Firmino raoni@linux.ibm.com Date: Mon, 1 Feb 2021 17:05:05 -0300 Subject: [PATCH] powerpc/64/signal: Fix regression in __kernel_sigtramp_rt64() semantics

Commit 0138ba5783ae ("powerpc/64/signal: Balance return predictor stack in signal trampoline") changed __kernel_sigtramp_rt64() VDSO and trampoline code, and introduced a regression in the way glibc's backtrace()[1] detects the signal-handler stack frame. Apart from the practical implications, __kernel_sigtramp_rt64() was a VDSO function with the semantics that it is a function you can call from userspace to end a signal handling. Now this semantics are no longer valid.

I believe the aforementioned change affects all releases since 5.9.

This patch tries to fix both the semantics and practical aspect of __kernel_sigtramp_rt64() returning it to the previous code, whilst keeping the intended behaviour of 0138ba5783ae by adding a new symbol to serve as the jump target from the kernel to the trampoline. Now the trampoline has two parts, a new entry point and the old return point.

[1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2021-January/223194.html

Fixes: 0138ba5783ae ("powerpc/64/signal: Balance return predictor stack in signal trampoline") Cc: stable@vger.kernel.org # v5.9+ Signed-off-by: Raoni Fassina Firmino raoni@linux.ibm.com Acked-by: Nicholas Piggin npiggin@gmail.com [mpe: Minor tweaks to change log formatting, add stable tag] Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20210201200505.iz46ubcizipnkcxe@work-tp

diff --git a/arch/powerpc/kernel/vdso64/sigtramp.S b/arch/powerpc/kernel/vdso64/sigtramp.S index bbf68cd01088..2d4067561293 100644 --- a/arch/powerpc/kernel/vdso64/sigtramp.S +++ b/arch/powerpc/kernel/vdso64/sigtramp.S @@ -15,11 +15,20 @@

.text

+/* + * __kernel_start_sigtramp_rt64 and __kernel_sigtramp_rt64 together + * are one function split in two parts. The kernel jumps to the former + * and the signal handler indirectly (by blr) returns to the latter. + * __kernel_sigtramp_rt64 needs to point to the return address so + * glibc can correctly identify the trampoline stack frame. + */ .balign 8 .balign IFETCH_ALIGN_BYTES -V_FUNCTION_BEGIN(__kernel_sigtramp_rt64) +V_FUNCTION_BEGIN(__kernel_start_sigtramp_rt64) .Lsigrt_start: bctrl /* call the handler */ +V_FUNCTION_END(__kernel_start_sigtramp_rt64) +V_FUNCTION_BEGIN(__kernel_sigtramp_rt64) addi r1, r1, __SIGNAL_FRAMESIZE li r0,__NR_rt_sigreturn sc diff --git a/arch/powerpc/kernel/vdso64/vdso64.lds.S b/arch/powerpc/kernel/vdso64/vdso64.lds.S index 6164d1a1ba11..2f3c359cacd3 100644 --- a/arch/powerpc/kernel/vdso64/vdso64.lds.S +++ b/arch/powerpc/kernel/vdso64/vdso64.lds.S @@ -131,4 +131,4 @@ VERSION /* * Make the sigreturn code visible to the kernel. */ -VDSO_sigtramp_rt64 = __kernel_sigtramp_rt64; +VDSO_sigtramp_rt64 = __kernel_start_sigtramp_rt64;