From a message from Joerg Pommnitz to the linux-kernel mailing list of 11 Nov 1996, edited.According to Linus Torvalds:
People interested in low-level scary stuff should take a look at the uaccess.h files for x86 or alpha, and be ready to spend some time just figuring out what it all does ;)
I am, and I did.
When a process runs in kernel mode, it often has to access user mode memory whose address has been passed by an untrusted program. To protect itself, the kernel has to verify this address.
In older versions of Linux, this was done with the
int verify_area(int type, const void * addr, unsigned long size)function.
This function verified, that the memory area starting at address addr and of size size was accessible for the operation specified in type (read or write). To do this, verify_read had to look up the virtual memory area (vma) that contained the address addr. In the normal case (correctly working program), this test was successful. It only failed for the (hopefully) rare, buggy program. In some kernel profiling tests, this normally unneeded verification used up a considerable amount of time.
To overcome this situation, Linus decided to let the virtual memory hardware present in every Linux capable CPU handle this test.
Whenever the kernel tries to access an address that is currently not accessible, the CPU generates a page fault exception and calls the page fault handler
void do_page_fault(struct pt_regs *regs, unsigned long error_code)in arch/i386/mm/fault.c. The parameters on the stack are set up by the low level assembly glue in arch/i386/kernel/entry.S. The parameter regs is a pointer to the saved registers on the stack, error_code contains a reason code for the exception.
do_page_fault first obtains the unaccessible address from the CPU control register CR2. If the address is within the virtual address space of the process, the fault probably occured, because the page was not swapped in, write protected or something similiar. However, we are interested in the other case: the address is not valid, there is no vma that contains this address. In this case, the kernel jumps to the bad_area label.
There it uses the address of the instruction that caused the exception (i.e. regs->eip) to find an address where the excecution can continue (fixup). If this search is successful, the fault handler modifies the return address (again regs->eip) and returns. The execution will continue at the address in fixup.
Since we jump to the the contents of fixup, fixup obviously points to executable code. This code is hidden inside the user access macros. I have picked the get_user macro defined in include/asm/uaccess.h as an example. The definition is somewhat hard to follow, so lets peek at the code generated by the preprocessor and the compiler. I selected the get_user call in drivers/char/console.c for a detailed examination.
The original code in console.c line 1405:
get_user(c, buf);
The preprocessor output (edited to become somewhat readable):
(
{
long __gu_err = - 14 , __gu_val = 0;
const __typeof__(*( ( buf ) )) *__gu_addr = ((buf));
if (((((0 + current_set[0])->tss.segment) == 0x18 ) ||
(((sizeof(*(buf))) <= 0xC0000000UL) &&
((unsigned long)(__gu_addr ) <= 0xC0000000UL - (sizeof(*(buf)))))))
do {
__gu_err = 0;
switch ((sizeof(*(buf)))) {
case 1:
__asm__ __volatile__(
"1: mov" "b" " %2,%" "b" "1\n"
"2:\n"
".section .fixup,\"ax\"\n"
"3: movl %3,%0\n"
" xor" "b" " %" "b" "1,%" "b" "1\n"
" jmp 2b\n"
".section __ex_table,\"a\"\n"
" .align 4\n"
" .long 1b,3b\n"
".text" : "=r"(__gu_err), "=q" (__gu_val): "m"((*(struct __large_struct *)
( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err )) ;
break;
case 2:
__asm__ __volatile__(
"1: mov" "w" " %2,%" "w" "1\n"
"2:\n"
".section .fixup,\"ax\"\n"
"3: movl %3,%0\n"
" xor" "w" " %" "w" "1,%" "w" "1\n"
" jmp 2b\n"
".section __ex_table,\"a\"\n"
" .align 4\n"
" .long 1b,3b\n"
".text" : "=r"(__gu_err), "=r" (__gu_val) : "m"((*(struct __large_struct *)
( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err ));
break;
case 4:
__asm__ __volatile__(
"1: mov" "l" " %2,%" "" "1\n"
"2:\n"
".section .fixup,\"ax\"\n"
"3: movl %3,%0\n"
" xor" "l" " %" "" "1,%" "" "1\n"
" jmp 2b\n"
".section __ex_table,\"a\"\n"
" .align 4\n" " .long 1b,3b\n"
".text" : "=r"(__gu_err), "=r" (__gu_val) : "m"((*(struct __large_struct *)
( __gu_addr )) ), "i"(- 14 ), "0"(__gu_err));
break;
default:
(__gu_val) = __get_user_bad();
}
} while (0) ;
((c)) = (__typeof__(*((buf))))__gu_val;
__gu_err;
}
);
WOW! Black GCC/assembly magic. This is impossible to follow, so lets
see what code gcc generates:
xorl %edx,%edx
movl current_set,%eax
cmpl $24,788(%eax)
je .L1424
cmpl $-1073741825,64(%esp)
ja .L1423
.L1424:
movl %edx,%eax
movl 64(%esp),%ebx
#APP
1: movb (%ebx),%dl /* this is the actual user access */
2:
.section .fixup,"ax"
3: movl $-14,%eax
xorb %dl,%dl
jmp 2b
.section __ex_table,"a"
.align 4
.long 1b,3b
.text
#NO_APP
.L1423:
movzbl %dl,%esi
The optimizer does a good job and gives us something we can actually
understand. Can we? The actual user access is quite obvious. Thanks
to the unified address space we can just access the address in user
memory. But what does the .section stuff do?
To understand this we have to look at the final kernel:
$ objdump --section-headers vmlinux
vmlinux: file format elf32-i386
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00098f40 c0100000 c0100000 00001000 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .fixup 000016bc c0198f40 c0198f40 00099f40 2**0
CONTENTS, ALLOC, LOAD, READONLY, CODE
2 .rodata 0000f127 c019a5fc c019a5fc 0009b5fc 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 __ex_table 000015c0 c01a9724 c01a9724 000aa724 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .data 0000ea58 c01abcf0 c01abcf0 000abcf0 2**4
CONTENTS, ALLOC, LOAD, DATA
5 .bss 00018e21 c01ba748 c01ba748 000ba748 2**2
ALLOC
6 .comment 00000ec4 00000000 00000000 000ba748 2**0
CONTENTS, READONLY
7 .note 00001068 00000ec4 00000ec4 000bb60c 2**0
CONTENTS, READONLY
There are obviously 2 non standard ELF sections in the generated object
file. But first we want to find out what happened to our code in the
final kernel executable:
$ objdump --disassemble --section=.text vmlinux c017e785 <do_con_write+c1> xorl %edx,%edx c017e787 <do_con_write+c3> movl 0xc01c7bec,%eax c017e78c <do_con_write+c8> cmpl $0x18,0x314(%eax) c017e793 <do_con_write+cf> je c017e79f <do_con_write+db> c017e795 <do_con_write+d1> cmpl $0xbfffffff,0x40(%esp,1) c017e79d <do_con_write+d9> ja c017e7a7 <do_con_write+e3> c017e79f <do_con_write+db> movl %edx,%eax c017e7a1 <do_con_write+dd> movl 0x40(%esp,1),%ebx c017e7a5 <do_con_write+e1> movb (%ebx),%dl c017e7a7 <do_con_write+e3> movzbl %dl,%esiThe whole user memory access is reduced to 10 x86 machine instructions. The instructions bracketed in the .section directives are not longer in the normal execution path. They are located in a different section of the executable file:
$ objdump --disassemble --section=.fixup vmlinux c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax c0199ffa <.fixup+10ba> xorb %dl,%dl c0199ffc <.fixup+10bc> jmp c017e7a7 <do_con_write+e3>And finally:
$ objdump --full-contents --section=__ex_table vmlinux c01aa7c4 93c017c0 e09f19c0 97c017c0 99c017c0 ................ c01aa7d4 f6c217c0 e99f19c0 a5e717c0 f59f19c0 ................ c01aa7e4 080a18c0 01a019c0 0a0a18c0 04a019c0 ................or in human readable byte order:
c01aa7c4 c017c093 c0199fe0 c017c097 c017c099 ................
c01aa7d4 c017c2f6 c0199fe9 c017e7a5 c0199ff5 ................
this is the interesting part!
c01aa7e4 c0180a08 c019a001 c0180a0a c019a004 ................
What happened? The assembly directives
.section .fixup,"ax" .section __ex_table,"a"told the assembler to move the following code to the specified sections in the ELF object file. So the instructions
3: movl $-14,%eax
xorb %dl,%dl
jmp 2b
ended up in the .fixup section of the object file and the addresses
.long 1b,3b
ended up in the __ex_table section of the object
file. 1b and 3b are local labels. The local label
1b (1b stands for next label 1 backward) is the address
of the instruction that might fault. In our case, the address of
the label 1b is c017e7a5:
1: movb (%ebx),%dl
c017e7a5 <do_con_write+e1> movb (%ebx),%dl
c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax
.section __ex_table,"a"
.align 4
.long 1b,3b
becomes the value pair
c01aa7d4 c017c2f6 c0199fe9 c017e7a5 c0199ff5 ................
^this is ^this is
1b 3b
c017e7a5,c0199ff5 in the exception table of the kernel.
In order for the function search_exception_table to find the exception table in the __ex_table section, it uses a linker feature: whenever the linker sees a section whose entire name is a valid C identifier, it creates the symbols __start_section and __stop_section delimiting the extents of the section. So search_exception_table brackets its search by __start___ex_table and __stop___ex_table
So, what actually happens if a fault from kernel mode with no suitable vma occurs?
c017e7a5 <do_con_write+e1> movb (%ebx),%dl
That's it, mostly. If you look at our example, you might ask why we set EAX to -EFAULT in the exception handler code. Well, the get_user macro actually returns a value: 0, if the user access was successful, -EFAULT on failure. Our original code did not test this return value, however the inline assembly code in get_user tries to return -EFAULT. GCC selected EAX to return this value.
Joerg Pommnitz | joerg@raleigh.ibm.com | Never attribute to malloc Mobile/Wireless | Dept UMRA | that which can be adequately Tel:(919)254-6397 | Office B502/E117 | explained by stupidity.