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Subject[PATCH v2 3/6] MIPS: scall: Handle seccomp filters which redirect syscalls
Date
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>
Cc: linux-mips@linux-mips.org
---

Changes in v2: None

arch/mips/kernel/scall32-o32.S | 11 +++++------
arch/mips/kernel/scall64-64.S | 3 +--
arch/mips/kernel/scall64-n32.S | 14 +++++++++-----
arch/mips/kernel/scall64-o32.S | 14 +++++++++-----
4 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/arch/mips/kernel/scall32-o32.S b/arch/mips/kernel/scall32-o32.S
index a56317444bda..6c1b5e085f2d 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 2b2dc14610d0..ede49e35d6a6 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 2bf5c8593d91..f75a6e19b902 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 c5b759e584c7..db224e7b995c 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
--
2.5.0
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