ARM: 9297/1: vfp: avoid unbalanced stack on 'success' return path

Commit c76c6c4ecbec0deb5 ("ARM: 9294/2: vfp: Fix broken softirq handling
with instrumentation enabled") updated the VFP exception entry logic to
go via a C function, so that we get the compiler's version of
local_bh_disable(), which may be instrumented, and isn't generally
callable from assembler.

However, this assumes that passing an alternative 'success' return
address works in C as it does in asm, and this is only the case if the C
calls in question are tail calls, as otherwise, the stack will need some
unwinding as well.

I have already sent patches to the list that replace most of the asm
logic with C code, and so it is preferable to have a minimal fix that
addresses the issue and can be backported along with the commit that it

fixes to v6.3 from v6.4. Hopefully, we can land the C conversion for v6.5.

So instead of passing the 'success' return address as a function
argument, pass the stack address from where to pop it so that both LR
and SP have the expected value.

Fixes: c76c6c4ecbec0deb5 ("ARM: 9294/2: vfp: Fix broken softirq handling with ...")
Reported-by: syzbot+d4b00edc2d0c910d4bf4@syzkaller.appspotmail.com
Tested-by: syzbot+d4b00edc2d0c910d4bf4@syzkaller.appspotmail.com
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

diff --git a/arch/arm/vfp/entry.S b/arch/arm/vfp/entry.S
index 7483ef8bccda394c0d21847e98b37597d8ec574a..62206ef250371cd35404b74ba3dd8c8ac51411fd 100644 (file)
--- a/arch/arm/vfp/entry.S
+++ b/arch/arm/vfp/entry.S
@@ -23,6 +23,9 @@
 @
 ENTRY(do_vfp)
        mov     r1, r10
-       mov     r3, r9
-       b       vfp_entry
+       str     lr, [sp, #-8]!
+       add     r3, sp, #4
+       str     r9, [r3]
+       bl      vfp_entry
+       ldr     pc, [sp], #8
 ENDPROC(do_vfp)

diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
index 4d8478264d82b3d2790b69b323118f8009c53787..a4610d0f321527cc86eee150254c6faeb5649dee 100644 (file)
--- a/arch/arm/vfp/vfphw.S
+++ b/arch/arm/vfp/vfphw.S
@@ -172,13 +172,14 @@ vfp_hw_state_valid:
                                        @ out before setting an FPEXC that
                                        @ stops us reading stuff
        VFPFMXR FPEXC, r1               @ Restore FPEXC last
+       mov     sp, r3                  @ we think we have handled things
+       pop     {lr}
        sub     r2, r2, #4              @ Retry current instruction - if Thumb
        str     r2, [sp, #S_PC]         @ mode it's two 16-bit instructions,
                                        @ else it's one 32-bit instruction, so
                                        @ always subtract 4 from the following
                                        @ instruction address.
 
-       mov     lr, r3                  @ we think we have handled things
 local_bh_enable_and_ret:
        adr     r0, .
        mov     r1, #SOFTIRQ_DISABLE_OFFSET
@@ -209,8 +210,9 @@ skip:
 
 process_exception:
        DBGSTR  "bounce"
+       mov     sp, r3                  @ setup for a return to the user code.
+       pop     {lr}
        mov     r2, sp                  @ nothing stacked - regdump is at TOS
-       mov     lr, r3                  @ setup for a return to the user code.
 
        @ Now call the C code to package up the bounce to the support code
        @   r0 holds the trigger instruction