Re: [PATCH 1/2] x86: entry_64.S: always allocate complete "struct pt_regs"

From: Borislav Petkov
Date: Thu Feb 12 2015 - 12:18:41 EST

On Wed, Feb 11, 2015 at 10:55:52PM +0100, Denys Vlasenko wrote:
> 64-bit code was using six stack slots less by not saving/restoring
> registers which are callee-preserved according to C ABI,
> and not allocating space for them.
> Only when syscall needed a complete "struct pt_regs",
> the complete area was allocated and filled in.
> As an additional twist, on interrupt entry a "slightly less truncated pt_regs"
> trick is used, to make nested interrupt stacks easier to unwind.
> This proved to be a source of significant obfuscation and subtle bugs.
> For example, stub_fork had to pop the return address,
> extend the struct, save registers, and push return address back. Ugly.
> ia32_ptregs_common pops return address and "returns" via jmp insn,
> throwing a wrench into CPU return stack cache.
> This patch changes code to always allocate a complete "struct pt_regs".
> The saving of registers is still done lazily.
> "Partial pt_regs" trick on interrupt stack is retained.
> Macros which manipulate "struct pt_regs" on stack are reworked:
> ALLOC_PT_GPREGS_ON_STACK allocates the structure.
> SAVE_C_REGS saves to it those registers which are clobbered by C code.
> SAVE_EXTRA_REGS saves to it all other registers.
> Corresponding RESTORE_* and REMOVE_PT_GPREGS_FROM_STACK macros reverse it.
> ia32_ptregs_common, stub_fork and friends lost their ugly dance with
> return pointer.
> LOAD_ARGS32 in ia32entry.S now uses symbolic stack offsets
> instead of magic numbers.
> error_entry and save_paranoid now use SAVE_C_REGS + SAVE_EXTRA_REGS
> instead of having it open-coded yet again.
> Patch was run-tested: 64-bit executables, 32-bit executables,
> strace works.
> Timing tests did not show measurable difference in 32-bit
> and 64-bit syscalls.
> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> CC: X86 ML <x86@xxxxxxxxxx>
> CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> CC: Will Drewry <wad@xxxxxxxxxxxx>
> CC: Kees Cook <keescook@xxxxxxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
> arch/x86/ia32/ia32entry.S | 47 +++----
> arch/x86/include/asm/calling.h | 222 ++++++++++++++++-----------------
> arch/x86/include/asm/irqflags.h | 4 +-
> arch/x86/include/uapi/asm/ptrace-abi.h | 1 -
> arch/x86/kernel/entry_64.S | 192 +++++++++++-----------------

You'd need to redo that patch against latest upstream because of changes
it is missing:

Andy's branch x86/entry should have them too:

arch/x86/kernel/entry_64.S: Assembler messages:
arch/x86/kernel/entry_64.S:760: Error: no such instruction: `restore_args 1,8,1'
make[2]: *** [arch/x86/kernel/entry_64.o] Error 1
make[1]: *** [arch/x86/kernel] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [arch/x86] Error 2
make: *** Waiting for unfinished jobs....


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