MIPS: scall: Handle seccomp filters which redirect syscalls

Commit d218af78492a ("MIPS: scall: Always run the seccomp syscall
filters") modified the syscall code to always call the seccomp filters,
but missed the case where a filter may redirect the syscall, as
revealed by the seccomp_bpf self test.

The syscall path now restores the syscall from the stack after the
filter rather than saving it locally. Syscall number checking and
syscall function table lookup is done after the filter may have run such
that redirected syscalls are also checked, and executed.

The regular path of syscall number checking and pointer lookup is also
made more consistent between ABIs with scall64-64.S being the reference.

With this patch in place, the seccomp_bpf self test now passes
TRACE_syscall.syscall_redirected and TRACE_syscall.syscall_dropped on
all MIPS ABIs.

Fixes: d218af78492a ("MIPS: scall: Always run the seccomp syscall filters")
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Eric B Munson <emunson@akamai.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mips@linux-mips.org
Cc: IMG-MIPSLinuxKerneldevelopers@imgtec.com
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12916/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

4 files changed, 24 insertions, 18 deletions

diff --git a/arch/mips/kernel/scall32-o32.S b/arch/mips/kernel/scall32-o32.S
index d01fe53a6638..c8e43e0c4066 100644
--- a/arch/mips/kernel/scall32-o32.S
+++ b/arch/mips/kernel/scall32-o32.S
@@ -35,7 +35,6 @@ NESTED(handle_sys, PT_SIZE, sp)
 
         lw      t1, PT_EPC(sp)          # skip syscall on return
 
-        subu    v0, v0, __NR_O32_Linux  # check syscall number
         addiu   t1, 4                   # skip to next instruction
         sw      t1, PT_EPC(sp)
 
@@ -89,6 +88,7 @@ loads_done:
         and     t0, t1
         bnez    t0, syscall_trace_entry # -> yes
 syscall_common:
+        subu    v0, v0, __NR_O32_Linux  # check syscall number
         sltiu   t0, v0, __NR_O32_Linux_syscalls + 1
         beqz    t0, illegal_syscall
 
@@ -118,24 +118,23 @@ o32_syscall_exit:
 
 syscall_trace_entry:
         SAVE_STATIC
-        move    s0, v0
         move    a0, sp
 
         /*
          * syscall number is in v0 unless we called syscall(__NR_###)
          * where the real syscall number is in a0
          */
-        addiu   a1, v0,  __NR_O32_Linux
-        bnez    v0, 1f /* __NR_syscall at offset 0 */
+        move    a1, v0
+        subu    t2, v0,  __NR_O32_Linux
+        bnez    t2, 1f /* __NR_syscall at offset 0 */
         lw      a1, PT_R4(sp)
 
 1:      jal     syscall_trace_enter
 
         bltz    v0, 1f                  # seccomp failed? Skip syscall
 
-        move    v0, s0                  # restore syscall
-
         RESTORE_STATIC
+        lw      v0, PT_R2(sp)           # Restore syscall (maybe modified)
         lw      a0, PT_R4(sp)           # Restore argument registers
         lw      a1, PT_R5(sp)
         lw      a2, PT_R6(sp)
diff --git a/arch/mips/kernel/scall64-64.S b/arch/mips/kernel/scall64-64.S
index 6b73ecc02597..e6ede125059f 100644
--- a/arch/mips/kernel/scall64-64.S
+++ b/arch/mips/kernel/scall64-64.S
@@ -82,15 +82,14 @@ n64_syscall_exit:
 
 syscall_trace_entry:
         SAVE_STATIC
-        move    s0, v0
         move    a0, sp
         move    a1, v0
         jal     syscall_trace_enter
 
         bltz    v0, 1f                  # seccomp failed? Skip syscall
 
-        move    v0, s0
         RESTORE_STATIC
+        ld      v0, PT_R2(sp)           # Restore syscall (maybe modified)
         ld      a0, PT_R4(sp)           # Restore argument registers
         ld      a1, PT_R5(sp)
         ld      a2, PT_R6(sp)
diff --git a/arch/mips/kernel/scall64-n32.S b/arch/mips/kernel/scall64-n32.S
index 71f99d5f7a06..9c0b387d6427 100644
--- a/arch/mips/kernel/scall64-n32.S
+++ b/arch/mips/kernel/scall64-n32.S
@@ -42,9 +42,6 @@ NESTED(handle_sysn32, PT_SIZE, sp)
 #endif
         beqz    t0, not_n32_scall
 
-        dsll    t0, v0, 3               # offset into table
-        ld      t2, (sysn32_call_table - (__NR_N32_Linux * 8))(t0)
-
         sd      a3, PT_R26(sp)          # save a3 for syscall restarting
 
         li      t1, _TIF_WORK_SYSCALL_ENTRY
@@ -53,6 +50,9 @@ NESTED(handle_sysn32, PT_SIZE, sp)
         bnez    t0, n32_syscall_trace_entry
 
 syscall_common:
+        dsll    t0, v0, 3               # offset into table
+        ld      t2, (sysn32_call_table - (__NR_N32_Linux * 8))(t0)
+
         jalr    t2                      # Do The Real Thing (TM)
 
         li      t0, -EMAXERRNO - 1      # error?
@@ -71,21 +71,25 @@ syscall_common:
 
 n32_syscall_trace_entry:
         SAVE_STATIC
-        move    s0, t2
         move    a0, sp
         move    a1, v0
         jal     syscall_trace_enter
 
         bltz    v0, 1f                  # seccomp failed? Skip syscall
 
-        move    t2, s0
         RESTORE_STATIC
+        ld      v0, PT_R2(sp)           # Restore syscall (maybe modified)
         ld      a0, PT_R4(sp)           # Restore argument registers
         ld      a1, PT_R5(sp)
         ld      a2, PT_R6(sp)
         ld      a3, PT_R7(sp)
         ld      a4, PT_R8(sp)
         ld      a5, PT_R9(sp)
+
+        dsubu   t2, v0, __NR_N32_Linux  # check (new) syscall number
+        sltiu   t0, t2, __NR_N32_Linux_syscalls + 1
+        beqz    t0, not_n32_scall
+
         j       syscall_common
 
 1:      j       syscall_exit
diff --git a/arch/mips/kernel/scall64-o32.S b/arch/mips/kernel/scall64-o32.S
index 91b43eea2d5a..f4f28b1580de 100644
--- a/arch/mips/kernel/scall64-o32.S
+++ b/arch/mips/kernel/scall64-o32.S
@@ -52,9 +52,6 @@ NESTED(handle_sys, PT_SIZE, sp)
         sll     a2, a2, 0
         sll     a3, a3, 0
 
-        dsll    t0, v0, 3               # offset into table
-        ld      t2, (sys32_call_table - (__NR_O32_Linux * 8))(t0)
-
         sd      a3, PT_R26(sp)          # save a3 for syscall restarting
 
         /*
@@ -88,6 +85,9 @@ loads_done:
         bnez    t0, trace_a_syscall
 
 syscall_common:
+        dsll    t0, v0, 3               # offset into table
+        ld      t2, (sys32_call_table - (__NR_O32_Linux * 8))(t0)
+
         jalr    t2                      # Do The Real Thing (TM)
 
         li      t0, -EMAXERRNO - 1      # error?
@@ -112,7 +112,6 @@ trace_a_syscall:
         sd      a6, PT_R10(sp)
         sd      a7, PT_R11(sp)          # For indirect syscalls
 
-        move    s0, t2                  # Save syscall pointer
         move    a0, sp
         /*
          * absolute syscall number is in v0 unless we called syscall(__NR_###)
@@ -133,8 +132,8 @@ trace_a_syscall:
 
         bltz    v0, 1f                  # seccomp failed? Skip syscall
 
-        move    t2, s0
         RESTORE_STATIC
+        ld      v0, PT_R2(sp)           # Restore syscall (maybe modified)
         ld      a0, PT_R4(sp)           # Restore argument registers
         ld      a1, PT_R5(sp)
         ld      a2, PT_R6(sp)
@@ -143,6 +142,11 @@ trace_a_syscall:
         ld      a5, PT_R9(sp)
         ld      a6, PT_R10(sp)
         ld      a7, PT_R11(sp)          # For indirect syscalls
+
+        dsubu   t0, v0, __NR_O32_Linux  # check (new) syscall number
+        sltiu   t0, t0, __NR_O32_Linux_syscalls + 1
+        beqz    t0, not_o32_scall
+
         j       syscall_common
 
 1:      j       syscall_exit