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Subject[PATCH 0/8] use struct pt_regs based syscall calling for x86-64
Date
Ingo,


On top of all the patches which remove in-kernel calls to syscall functions
merged in commit 642e7fd23353, 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.

This approach avoids leaking random user-provided register content down
the call chain. Therefore, the seventh 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.


Changes since the series sent out to linux-kernel on March 30th:

all patches:
- rebase on top of commit 642e7fd23353

several patches:
- further extend and fix commentary; spelling fixes (e.g., nospec, 64-bit,
32-bit)

patch 3:
- do not clobber regs->dx on sys_getcpu() vsyscall

patch 5:
- rename __sys32_ia32_*() stubs to __sys_ia32_*()
- do not generate __sys_ia32_*() syscall table entries automatically, but
have them explicitely in arch/x86/entry/syscalls/syscall_32.tbl
- this means that there is no need to redefine SYSCALL_DEFINE0
- rename compat_sys_*() to __compat_sys_ia32_*(), as the calling convention
is different to "generic" compat_sys_*() [but see below]

patch 8: (your call...)
- introduce new patch 8: rename sys_*() to __sys_x86_*() -- while this
avoids symbol space overlap per your request, it doesn't improve the
code readibility by much. Moreover, if other architectures switch to
this syscall calling convention, there is no real "default" calling
convention any more. Therefore, I'd suggest *NOT* to apply this patch.

Thanks,
Dominik

Dominik Brodowski (7):
syscalls: introduce CONFIG_ARCH_HAS_SYSCALL_WRAPPER
syscalls/x86: use struct pt_regs based syscall calling for 64-bit
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
syscalls/x86: rename struct pt_regs-based sys_*() to __sys_x86_*()

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_32.tbl | 724 +++++++++++++++++----------------
arch/x86/entry/syscalls/syscall_64.tbl | 712 ++++++++++++++++----------------
arch/x86/entry/vsyscall/vsyscall_64.c | 18 +-
arch/x86/include/asm/syscall.h | 4 +
arch/x86/include/asm/syscall_wrapper.h | 197 +++++++++
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, 1054 insertions(+), 748 deletions(-)
create mode 100644 arch/x86/include/asm/syscall_wrapper.h

--
2.16.3

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