arm64: entry: avoid writing lr explicitly for constructing return paths

Using an explicit adr instruction to set the link register to point at
ret_fast_syscall/ret_to_user can defeat branch and return stack predictors.

Instead, use the standard calling instructions (bl, blr) and have an
unconditional branch as the following instruction.

Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 726b910..2cebe56 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -455,8 +455,8 @@ el0_da:
        bic     x0, x26, #(0xff << 56)
        mov     x1, x25
        mov     x2, sp
-       adr     lr, ret_to_user
-       b       do_mem_abort
+       bl      do_mem_abort
+       b       ret_to_user
 el0_ia:
        /*
         * Instruction abort handling
@@ -468,8 +468,8 @@ el0_ia:
        mov     x0, x26
        orr     x1, x25, #1 << 24               // use reserved ISS bit for instruction aborts
        mov     x2, sp
-       adr     lr, ret_to_user
-       b       do_mem_abort
+       bl      do_mem_abort
+       b       ret_to_user
 el0_fpsimd_acc:
        /*
         * Floating Point or Advanced SIMD access
@@ -478,8 +478,8 @@ el0_fpsimd_acc:
        ct_user_exit
        mov     x0, x25
        mov     x1, sp
-       adr     lr, ret_to_user
-       b       do_fpsimd_acc
+       bl      do_fpsimd_acc
+       b       ret_to_user
 el0_fpsimd_exc:
        /*
         * Floating Point or Advanced SIMD exception
@@ -488,8 +488,8 @@ el0_fpsimd_exc:
        ct_user_exit
        mov     x0, x25
        mov     x1, sp
-       adr     lr, ret_to_user
-       b       do_fpsimd_exc
+       bl      do_fpsimd_exc
+       b       ret_to_user
 el0_sp_pc:
        /*
         * Stack or PC alignment exception handling
@@ -500,8 +500,8 @@ el0_sp_pc:
        mov     x0, x26
        mov     x1, x25
        mov     x2, sp
-       adr     lr, ret_to_user
-       b       do_sp_pc_abort
+       bl      do_sp_pc_abort
+       b       ret_to_user
 el0_undef:
        /*
         * Undefined instruction
@@ -510,8 +510,8 @@ el0_undef:
        enable_dbg_and_irq
        ct_user_exit
        mov     x0, sp
-       adr     lr, ret_to_user
-       b       do_undefinstr
+       bl      do_undefinstr
+       b       ret_to_user
 el0_dbg:
        /*
         * Debug exception handling
@@ -530,8 +530,8 @@ el0_inv:
        mov     x0, sp
        mov     x1, #BAD_SYNC
        mrs     x2, esr_el1
-       adr     lr, ret_to_user
-       b       bad_mode
+       bl      bad_mode
+       b       ret_to_user
 ENDPROC(el0_sync)
 
        .align  6
@@ -653,14 +653,15 @@ el0_svc_naked:                                    // compat entry point
        ldr     x16, [tsk, #TI_FLAGS]           // check for syscall hooks
        tst     x16, #_TIF_SYSCALL_WORK
        b.ne    __sys_trace
-       adr     lr, ret_fast_syscall            // return address
        cmp     scno, sc_nr                     // check upper syscall limit
        b.hs    ni_sys
        ldr     x16, [stbl, scno, lsl #3]       // address in the syscall table
-       br      x16                             // call sys_* routine
+       blr     x16                             // call sys_* routine
+       b       ret_fast_syscall
 ni_sys:
        mov     x0, sp
-       b       do_ni_syscall
+       bl      do_ni_syscall
+       b       ret_fast_syscall
 ENDPROC(el0_svc)
 
        /*
@@ -670,17 +671,16 @@ ENDPROC(el0_svc)
 __sys_trace:
        mov     x0, sp
        bl      syscall_trace_enter
-       adr     lr, __sys_trace_return          // return address
        uxtw    scno, w0                        // syscall number (possibly new)
        mov     x1, sp                          // pointer to regs
        cmp     scno, sc_nr                     // check upper syscall limit
-       b.hs    ni_sys
+       b.hs    __ni_sys_trace
        ldp     x0, x1, [sp]                    // restore the syscall args
        ldp     x2, x3, [sp, #S_X2]
        ldp     x4, x5, [sp, #S_X4]
        ldp     x6, x7, [sp, #S_X6]
        ldr     x16, [stbl, scno, lsl #3]       // address in the syscall table
-       br      x16                             // call sys_* routine
+       blr     x16                             // call sys_* routine
 
 __sys_trace_return:
        str     x0, [sp]                        // save returned x0
@@ -688,6 +688,11 @@ __sys_trace_return:
        bl      syscall_trace_exit
        b       ret_to_user
 
+__ni_sys_trace:
+       mov     x0, sp
+       bl      do_ni_syscall
+       b       __sys_trace_return
+
 /*
  * Special system call wrappers.
  */