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1 Kernel level exception handling in Linux
2 Commentary by Joerg Pommnitz <joerg@raleigh.ibm.com>
3
4When a process runs in kernel mode, it often has to access user
5mode memory whose address has been passed by an untrusted program.
6To protect itself the kernel has to verify this address.
7
8In older versions of Linux this was done with the
9int verify_area(int type, const void * addr, unsigned long size)
10function (which has since been replaced by access_ok()).
11
12This function verified that the memory area starting at address
13'addr' and of size 'size' was accessible for the operation specified
14in type (read or write). To do this, verify_read had to look up the
15virtual memory area (vma) that contained the address addr. In the
16normal case (correctly working program), this test was successful.
17It only failed for a few buggy programs. In some kernel profiling
18tests, this normally unneeded verification used up a considerable
19amount of time.
20
21To overcome this situation, Linus decided to let the virtual memory
22hardware present in every Linux-capable CPU handle this test.
23
24How does this work?
25
26Whenever the kernel tries to access an address that is currently not
27accessible, the CPU generates a page fault exception and calls the
28page fault handler
29
30void do_page_fault(struct pt_regs *regs, unsigned long error_code)
31
32in arch/x86/mm/fault.c. The parameters on the stack are set up by
33the low level assembly glue in arch/x86/kernel/entry_32.S. The parameter
34regs is a pointer to the saved registers on the stack, error_code
35contains a reason code for the exception.
36
37do_page_fault first obtains the unaccessible address from the CPU
38control register CR2. If the address is within the virtual address
39space of the process, the fault probably occurred, because the page
40was not swapped in, write protected or something similar. However,
41we are interested in the other case: the address is not valid, there
42is no vma that contains this address. In this case, the kernel jumps
43to the bad_area label.
44
45There it uses the address of the instruction that caused the exception
46(i.e. regs->eip) to find an address where the execution can continue
47(fixup). If this search is successful, the fault handler modifies the
48return address (again regs->eip) and returns. The execution will
49continue at the address in fixup.
50
51Where does fixup point to?
52
53Since we jump to the contents of fixup, fixup obviously points
54to executable code. This code is hidden inside the user access macros.
55I have picked the get_user macro defined in arch/x86/include/asm/uaccess.h
56as an example. The definition is somewhat hard to follow, so let's peek at
57the code generated by the preprocessor and the compiler. I selected
58the get_user call in drivers/char/sysrq.c for a detailed examination.
59
60The original code in sysrq.c line 587:
61 get_user(c, buf);
62
63The preprocessor output (edited to become somewhat readable):
64
65(
66 {
67 long __gu_err = - 14 , __gu_val = 0;
68 const __typeof__(*( ( buf ) )) *__gu_addr = ((buf));
69 if (((((0 + current_set[0])->tss.segment) == 0x18 ) ||
70 (((sizeof(*(buf))) <= 0xC0000000UL) &&
71 ((unsigned long)(__gu_addr ) <= 0xC0000000UL - (sizeof(*(buf)))))))
72 do {
73 __gu_err = 0;
74 switch ((sizeof(*(buf)))) {
75 case 1:
76 __asm__ __volatile__(
77 "1: mov" "b" " %2,%" "b" "1\n"
78 "2:\n"
79 ".section .fixup,\"ax\"\n"
80 "3: movl %3,%0\n"
81 " xor" "b" " %" "b" "1,%" "b" "1\n"
82 " jmp 2b\n"
83 ".section __ex_table,\"a\"\n"
84 " .align 4\n"
85 " .long 1b,3b\n"
86 ".text" : "=r"(__gu_err), "=q" (__gu_val): "m"((*(struct __large_struct *)
87 ( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err )) ;
88 break;
89 case 2:
90 __asm__ __volatile__(
91 "1: mov" "w" " %2,%" "w" "1\n"
92 "2:\n"
93 ".section .fixup,\"ax\"\n"
94 "3: movl %3,%0\n"
95 " xor" "w" " %" "w" "1,%" "w" "1\n"
96 " jmp 2b\n"
97 ".section __ex_table,\"a\"\n"
98 " .align 4\n"
99 " .long 1b,3b\n"
100 ".text" : "=r"(__gu_err), "=r" (__gu_val) : "m"((*(struct __large_struct *)
101 ( __gu_addr )) ), "i"(- 14 ), "0"( __gu_err ));
102 break;
103 case 4:
104 __asm__ __volatile__(
105 "1: mov" "l" " %2,%" "" "1\n"
106 "2:\n"
107 ".section .fixup,\"ax\"\n"
108 "3: movl %3,%0\n"
109 " xor" "l" " %" "" "1,%" "" "1\n"
110 " jmp 2b\n"
111 ".section __ex_table,\"a\"\n"
112 " .align 4\n" " .long 1b,3b\n"
113 ".text" : "=r"(__gu_err), "=r" (__gu_val) : "m"((*(struct __large_struct *)
114 ( __gu_addr )) ), "i"(- 14 ), "0"(__gu_err));
115 break;
116 default:
117 (__gu_val) = __get_user_bad();
118 }
119 } while (0) ;
120 ((c)) = (__typeof__(*((buf))))__gu_val;
121 __gu_err;
122 }
123);
124
125WOW! Black GCC/assembly magic. This is impossible to follow, so let's
126see what code gcc generates:
127
128 > xorl %edx,%edx
129 > movl current_set,%eax
130 > cmpl $24,788(%eax)
131 > je .L1424
132 > cmpl $-1073741825,64(%esp)
133 > ja .L1423
134 > .L1424:
135 > movl %edx,%eax
136 > movl 64(%esp),%ebx
137 > #APP
138 > 1: movb (%ebx),%dl /* this is the actual user access */
139 > 2:
140 > .section .fixup,"ax"
141 > 3: movl $-14,%eax
142 > xorb %dl,%dl
143 > jmp 2b
144 > .section __ex_table,"a"
145 > .align 4
146 > .long 1b,3b
147 > .text
148 > #NO_APP
149 > .L1423:
150 > movzbl %dl,%esi
151
152The optimizer does a good job and gives us something we can actually
153understand. Can we? The actual user access is quite obvious. Thanks
154to the unified address space we can just access the address in user
155memory. But what does the .section stuff do?????
156
157To understand this we have to look at the final kernel:
158
159 > objdump --section-headers vmlinux
160 >
161 > vmlinux: file format elf32-i386
162 >
163 > Sections:
164 > Idx Name Size VMA LMA File off Algn
165 > 0 .text 00098f40 c0100000 c0100000 00001000 2**4
166 > CONTENTS, ALLOC, LOAD, READONLY, CODE
167 > 1 .fixup 000016bc c0198f40 c0198f40 00099f40 2**0
168 > CONTENTS, ALLOC, LOAD, READONLY, CODE
169 > 2 .rodata 0000f127 c019a5fc c019a5fc 0009b5fc 2**2
170 > CONTENTS, ALLOC, LOAD, READONLY, DATA
171 > 3 __ex_table 000015c0 c01a9724 c01a9724 000aa724 2**2
172 > CONTENTS, ALLOC, LOAD, READONLY, DATA
173 > 4 .data 0000ea58 c01abcf0 c01abcf0 000abcf0 2**4
174 > CONTENTS, ALLOC, LOAD, DATA
175 > 5 .bss 00018e21 c01ba748 c01ba748 000ba748 2**2
176 > ALLOC
177 > 6 .comment 00000ec4 00000000 00000000 000ba748 2**0
178 > CONTENTS, READONLY
179 > 7 .note 00001068 00000ec4 00000ec4 000bb60c 2**0
180 > CONTENTS, READONLY
181
182There are obviously 2 non standard ELF sections in the generated object
183file. But first we want to find out what happened to our code in the
184final kernel executable:
185
186 > objdump --disassemble --section=.text vmlinux
187 >
188 > c017e785 <do_con_write+c1> xorl %edx,%edx
189 > c017e787 <do_con_write+c3> movl 0xc01c7bec,%eax
190 > c017e78c <do_con_write+c8> cmpl $0x18,0x314(%eax)
191 > c017e793 <do_con_write+cf> je c017e79f <do_con_write+db>
192 > c017e795 <do_con_write+d1> cmpl $0xbfffffff,0x40(%esp,1)
193 > c017e79d <do_con_write+d9> ja c017e7a7 <do_con_write+e3>
194 > c017e79f <do_con_write+db> movl %edx,%eax
195 > c017e7a1 <do_con_write+dd> movl 0x40(%esp,1),%ebx
196 > c017e7a5 <do_con_write+e1> movb (%ebx),%dl
197 > c017e7a7 <do_con_write+e3> movzbl %dl,%esi
198
199The whole user memory access is reduced to 10 x86 machine instructions.
200The instructions bracketed in the .section directives are no longer
201in the normal execution path. They are located in a different section
202of the executable file:
203
204 > objdump --disassemble --section=.fixup vmlinux
205 >
206 > c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax
207 > c0199ffa <.fixup+10ba> xorb %dl,%dl
208 > c0199ffc <.fixup+10bc> jmp c017e7a7 <do_con_write+e3>
209
210And finally:
211 > objdump --full-contents --section=__ex_table vmlinux
212 >
213 > c01aa7c4 93c017c0 e09f19c0 97c017c0 99c017c0 ................
214 > c01aa7d4 f6c217c0 e99f19c0 a5e717c0 f59f19c0 ................
215 > c01aa7e4 080a18c0 01a019c0 0a0a18c0 04a019c0 ................
216
217or in human readable byte order:
218
219 > c01aa7c4 c017c093 c0199fe0 c017c097 c017c099 ................
220 > c01aa7d4 c017c2f6 c0199fe9 c017e7a5 c0199ff5 ................
221 ^^^^^^^^^^^^^^^^^
222 this is the interesting part!
223 > c01aa7e4 c0180a08 c019a001 c0180a0a c019a004 ................
224
225What happened? The assembly directives
226
227.section .fixup,"ax"
228.section __ex_table,"a"
229
230told the assembler to move the following code to the specified
231sections in the ELF object file. So the instructions
2323: movl $-14,%eax
233 xorb %dl,%dl
234 jmp 2b
235ended up in the .fixup section of the object file and the addresses
236 .long 1b,3b
237ended up in the __ex_table section of the object file. 1b and 3b
238are local labels. The local label 1b (1b stands for next label 1
239backward) is the address of the instruction that might fault, i.e.
240in our case the address of the label 1 is c017e7a5:
241the original assembly code: > 1: movb (%ebx),%dl
242and linked in vmlinux : > c017e7a5 <do_con_write+e1> movb (%ebx),%dl
243
244The local label 3 (backwards again) is the address of the code to handle
245the fault, in our case the actual value is c0199ff5:
246the original assembly code: > 3: movl $-14,%eax
247and linked in vmlinux : > c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax
248
249The assembly code
250 > .section __ex_table,"a"
251 > .align 4
252 > .long 1b,3b
253
254becomes the value pair
255 > c01aa7d4 c017c2f6 c0199fe9 c017e7a5 c0199ff5 ................
256 ^this is ^this is
257 1b 3b
258c017e7a5,c0199ff5 in the exception table of the kernel.
259
260So, what actually happens if a fault from kernel mode with no suitable
261vma occurs?
262
2631.) access to invalid address:
264 > c017e7a5 <do_con_write+e1> movb (%ebx),%dl
2652.) MMU generates exception
2663.) CPU calls do_page_fault
2674.) do page fault calls search_exception_table (regs->eip == c017e7a5);
2685.) search_exception_table looks up the address c017e7a5 in the
269 exception table (i.e. the contents of the ELF section __ex_table)
270 and returns the address of the associated fault handle code c0199ff5.
2716.) do_page_fault modifies its own return address to point to the fault
272 handle code and returns.
2737.) execution continues in the fault handling code.
2748.) 8a) EAX becomes -EFAULT (== -14)
275 8b) DL becomes zero (the value we "read" from user space)
276 8c) execution continues at local label 2 (address of the
277 instruction immediately after the faulting user access).
278
279The steps 8a to 8c in a certain way emulate the faulting instruction.
280
281That's it, mostly. If you look at our example, you might ask why
282we set EAX to -EFAULT in the exception handler code. Well, the
283get_user macro actually returns a value: 0, if the user access was
284successful, -EFAULT on failure. Our original code did not test this
285return value, however the inline assembly code in get_user tries to
286return -EFAULT. GCC selected EAX to return this value.
287
288NOTE:
289Due to the way that the exception table is built and needs to be ordered,
290only use exceptions for code in the .text section. Any other section
291will cause the exception table to not be sorted correctly, and the
292exceptions will fail.
generated by cgit v1.2.2 (git 2.25.0) at 2025-12-31 19:57:13 -0500