Re: [PATCH] proc: fix PIE proc-empty-vm, proc-pid-vm tests

[Mirsad Goran Todorovac]

Hi,

On 06. 01. 2023. 20:30, Alexey Dobriyan wrote:
> vsyscall detection code uses direct call to the beginning of
> the vsyscall page:
>
> 	asm ("call %P0" :: "i" (0xffffffffff600000))
>
> It generates "call rel32" instruction but it is not relocated if binary
> is PIE, so binary segfaults into random userspace address and vsyscall
> page status is detected incorrectly.
>
> Do more direct:
>
> 	asm ("call *%rax")
>
> which doesn't do need any relocaltions.
>
> Mark g_vsyscall as volatile for a good measure, I didn't find instruction
> setting it to 0. Now the code is obviously correct:
>
> 	xor	eax, eax
> 	mov	rdi, rbp
> 	mov	rsi, rbp
> 	mov	DWORD PTR [rip+0x2d15], eax      # g_vsyscall = 0
> 	mov	rax, 0xffffffffff600000
> 	call	rax
> 	mov	DWORD PTR [rip+0x2d02], 1        # g_vsyscall = 1
> 	mov	eax, DWORD PTR ds:0xffffffffff600000
> 	mov	DWORD PTR [rip+0x2cf1], 2        # g_vsyscall = 2
> 	mov	edi, [rip+0x2ceb]                # exit(g_vsyscall)
> 	call	exit
>
> Note: fixed proc-empty-vm test oopses 5.19.0-28-generic kernel
> 	but this is separate story.
>
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@xxxxxxxxxxxx>
> ---
>
>   tools/testing/selftests/proc/proc-empty-vm.c |   12 +++++++-----
>   tools/testing/selftests/proc/proc-pid-vm.c   |    9 +++++----
>   2 files changed, 12 insertions(+), 9 deletions(-)
>
> --- a/tools/testing/selftests/proc/proc-empty-vm.c
> +++ b/tools/testing/selftests/proc/proc-empty-vm.c
> @@ -25,6 +25,7 @@
>   #undef NDEBUG
>   #include <assert.h>
>   #include <errno.h>
> +#include <stdint.h>
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <string.h>
> @@ -41,7 +42,7 @@
>    * 1: vsyscall VMA is --xp		vsyscall=xonly
>    * 2: vsyscall VMA is r-xp		vsyscall=emulate
>    */
> -static int g_vsyscall;
> +static volatile int g_vsyscall;
>   static const char *g_proc_pid_maps_vsyscall;
>   static const char *g_proc_pid_smaps_vsyscall;
>   
> @@ -147,11 +148,12 @@ static void vsyscall(void)
>   
>   		g_vsyscall = 0;
>   		/* gettimeofday(NULL, NULL); */
> +		uint64_t rax = 0xffffffffff600000;
>   		asm volatile (
> -			"call %P0"
> -			:
> -			: "i" (0xffffffffff600000), "D" (NULL), "S" (NULL)
nt> -			: "rax", "rcx", "r11"
> +			"call *%[rax]"
> +			: [rax] "+a" (rax)
> +			: "D" (NULL), "S" (NULL)
> +			: "rcx", "r11"
>   		);
>   
>   		g_vsyscall = 1;
> --- a/tools/testing/selftests/proc/proc-pid-vm.c
> +++ b/tools/testing/selftests/proc/proc-pid-vm.c
> @@ -257,11 +257,12 @@ static void vsyscall(void)
>   
>   		g_vsyscall = 0;
>   		/* gettimeofday(NULL, NULL); */
> +		uint64_t rax = 0xffffffffff600000;
>   		asm volatile (
> -			"call %P0"
> -			:
> -			: "i" (0xffffffffff600000), "D" (NULL), "S" (NULL)
> -			: "rax", "rcx", "r11"
> +			"call *%[rax]"
> +			: [rax] "+a" (rax)
> +			: "D" (NULL), "S" (NULL)
> +			: "rcx", "r11"
>   		);
>   
>   		g_vsyscall = 1;

I can confirm that the patch fixed the core dump in the exact environment that
used to reproduce the bug.

Apparently, it seems that gcc 12.2.0 -O2 optimiser on Ubuntu 22.10 kinetic kudu
did some new creative stuff to Alexey's code. For someone interested, I have saved the
assembly with and w/o -O2 ...

However, I have just found some spurious bug in proc-uptime-001.

But, this is another story ...

Thanks,
Mirsad

--
Mirsad Goran Todorovac
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu
--
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia
The European Union