[PATCH 0/7] use struct pt_regs based syscall calling for x86-64

Dominik Brodowski
2018-03-30T09:40:47Z

On top of all the patches which remove in-kernel calls to syscall functions
sent out yesterday[*[, it now becomes easy for achitectures to re-define the
syscall calling convention. For x86, this may be used to merely decode those
entries from struct pt_regs which are needed for a specific syscall.

[*] http://lkml.kernel.org/r/20180329112426.23043-1-linux@dominikbrodowski.net

This approach avoids leaking random user-provided register content down
the call chain. Therefore, the last patch of this series extends the
register clearing in the entry path to a few more registers.

To exemplify: sys_recv() is a classic 4-parameter syscall. For this syscall,
the DEFINE_SYSCALL macro creates the following stub:

asmlinkage long sys_recv(struct pt_regs *regs)
{
return SyS_recv(regs->di, regs->si, regs->dx, regs->r10);
}

The assembly of that function then becomes, in slightly reordered fashion:

<sys_recv>:
callq <__fentry__>

/* decode regs->di, ->si, ->dx and ->r10 */
mov 0x70(%rdi),%rdi
mov 0x68(%rdi),%rsi
mov 0x60(%rdi),%rdx
mov 0x38(%rdi),%rcx

[ SyS_recv() is inlined here by the compiler, as it is tiny ]
/* clear %r9 and %r8, the 5th and 6th args */
xor %r9d,%r9d
xor %r8d,%r8d

/* do the actual work */
callq __sys_recvfrom

/* cleanup and return */
cltq
retq

For IA32_EMULATION and X32, additional care needs to be taken as they use
different registers to pass parameters to syscalls; vsyscalls need to be
modified to use this new calling convention as well.

This actual conversion of x86 syscalls is heavily based on a proof-of-concept
by Linus[*]. This patchset here differs, for example, as it provides a generic
config symbol ARCH_HAS_SYSCALL_WRAPPER, introduces <asm/syscall_wrapper.h>,
splits up the patch into several parts, and adds the actual register clearing.

[*] Accessible at
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git WIP-syscall
It contains an additional patch
x86: avoid per-cpu system call trampoline
which is not included in my series as it addresses a different
issue, but may be of interest to the x86 maintainers as well.

Compared to v4.16-rc5 baseline and on a random kernel config, these patches
(in combination with the large do-not-call-syscalls-in-the-kernel series)
lead to a minisculue increase in text (+0.005%) and data (+0.11%) size on a
pure 64bit system,

text data bss dec hex filename
18853337 9535476 938380 29327193 1bf7f59 vmlinux-orig
18854227 9546100 938380 29338707 1bfac53 vmlinux,

with IA32_EMULATION and X32 enabled, the situation is just a little bit worse
for text size (+0.009%) and data (+0.38%) size.

text data bss dec hex filename
18902496 9603676 938444 29444616 1c14a08 vmlinux-orig
18904136 9640604 938444 29483184 1c1e0b0 vmlinux.

The 64bit part of this series has worked flawlessly on my local system for a
few weeks. IA32_EMULATION and x32 has passed some basic testing as well, but
has not yet been tested as extensively as x86-64. Pure i386 kernels are left
as-is, as they use a different asmlinkage anyway.

A few questions remain, from important stuff to bikeshedding:

1) Is it acceptable to pass the existing struct pt_regs to the sys_*()
kernel functions in emulate_vsyscall(), or should it use a hand-crafted
struct pt_regs instead?

2) Is it the right approach to generate the __sys32_ia32_*() names to
include in the syscall table on-the-fly, or should they all be listed
in arch/x86/entry/syscalls/syscall_32.tbl ?

3) I have chosen to name the default 64-bit syscall stub sys_*(), same as
the "normal" syscall, and the IA32_EMULATION compat syscall stub
compat_sys_*(), same as the "normal" compat syscall. Though this
might cause some confusion, as the "same" function uses a different
calling convention and different parameters on x86, it has the
advantages that
- the kernel *has* a function sys_*() implementing the syscall,
so those curious in stack traces etc. will find it in plain
sight,
- it is easier to handle in the syscall table generation, and
- error injection works the same.

The whole series is available at

https://git.kernel.org/pub/scm/linux/kernel/git/brodo/linux.git syscalls-WIP

Thanks,
Dominik

Dominik Brodowski (6):
syscalls: introduce CONFIG_ARCH_HAS_SYSCALL_WRAPPER
syscalls/x86: use struct pt_regs based syscall calling for 64bit
syscalls
syscalls: prepare ARCH_HAS_SYSCALL_WRAPPER for compat syscalls
syscalls/x86: use struct pt_regs based syscall calling for
IA32_EMULATION and x32
syscalls/x86: unconditionally enable struct pt_regs based syscalls on
x86_64
x86/entry/64: extend register clearing on syscall entry to lower
registers

Linus Torvalds (1):
x86: don't pointlessly reload the system call number

arch/x86/Kconfig | 1 +
arch/x86/entry/calling.h | 2 +
arch/x86/entry/common.c | 20 ++--
arch/x86/entry/entry_64.S | 3 +-
arch/x86/entry/entry_64_compat.S | 6 ++
arch/x86/entry/syscall_32.c | 15 ++-
arch/x86/entry/syscall_64.c | 6 +-
arch/x86/entry/syscalls/syscall_64.tbl | 74 ++++++-------
arch/x86/entry/syscalls/syscalltbl.sh | 8 ++
arch/x86/entry/vsyscall/vsyscall_64.c | 14 +--
arch/x86/include/asm/syscall.h | 4 +
arch/x86/include/asm/syscall_wrapper.h | 189 +++++++++++++++++++++++++++++++++
arch/x86/include/asm/syscalls.h | 17 ++-
include/linux/compat.h | 22 ++++
include/linux/syscalls.h | 25 ++++-
init/Kconfig | 10 ++
kernel/sys_ni.c | 10 ++
kernel/time/posix-stubs.c | 10 ++
18 files changed, 365 insertions(+), 71 deletions(-)
create mode 100644 arch/x86/include/asm/syscall_wrapper.h

--
2.16.3