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Change-Id: I8a9ee2aea93cd29c52c847d0ce33091a73ae6afe
diff --git a/Documentation/x86/exception-tables.txt b/Documentation/x86/exception-tables.txt
new file mode 100644
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+ Kernel level exception handling in Linux
+ Commentary by Joerg Pommnitz <joerg@raleigh.ibm.com>
+
+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+function (which has since been replaced by access_ok()).
+
+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 a few buggy programs. 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.
+
+How does this work?
+
+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/x86/mm/fault.c. The parameters on the stack are set up by
+the low level assembly glue in arch/x86/kernel/entry_32.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 occurred, because the page
+was not swapped in, write protected or something similar. 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 execution 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.
+
+Where does fixup point to?
+
+Since we jump to 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 arch/x86/include/asm/uaccess.h
+as an example. The definition is somewhat hard to follow, so let's peek at
+the code generated by the preprocessor and the compiler. I selected
+the get_user call in drivers/char/sysrq.c for a detailed examination.
+
+The original code in sysrq.c line 587:
+ 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 let's
+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,%esi
+
+The whole user memory access is reduced to 10 x86 machine instructions.
+The instructions bracketed in the .section directives are no 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, i.e.
+in our case the address of the label 1 is c017e7a5:
+the original assembly code: > 1: movb (%ebx),%dl
+and linked in vmlinux : > c017e7a5 <do_con_write+e1> movb (%ebx),%dl
+
+The local label 3 (backwards again) is the address of the code to handle
+the fault, in our case the actual value is c0199ff5:
+the original assembly code: > 3: movl $-14,%eax
+and linked in vmlinux : > c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax
+
+The assembly code
+ > .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.
+
+So, what actually happens if a fault from kernel mode with no suitable
+vma occurs?
+
+1.) access to invalid address:
+ > c017e7a5 <do_con_write+e1> movb (%ebx),%dl
+2.) MMU generates exception
+3.) CPU calls do_page_fault
+4.) do page fault calls search_exception_table (regs->eip == c017e7a5);
+5.) search_exception_table looks up the address c017e7a5 in the
+ exception table (i.e. the contents of the ELF section __ex_table)
+ and returns the address of the associated fault handle code c0199ff5.
+6.) do_page_fault modifies its own return address to point to the fault
+ handle code and returns.
+7.) execution continues in the fault handling code.
+8.) 8a) EAX becomes -EFAULT (== -14)
+ 8b) DL becomes zero (the value we "read" from user space)
+ 8c) execution continues at local label 2 (address of the
+ instruction immediately after the faulting user access).
+
+The steps 8a to 8c in a certain way emulate the faulting instruction.
+
+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.
+
+NOTE:
+Due to the way that the exception table is built and needs to be ordered,
+only use exceptions for code in the .text section. Any other section
+will cause the exception table to not be sorted correctly, and the
+exceptions will fail.