1  // SPDX-License-Identifier: GPL-2.0-or-later
2  #include <string.h>
3  
4  #include <objtool/special.h>
5  #include <objtool/builtin.h>
6  
7  #define X86_FEATURE_POPCNT (4 * 32 + 23)
8  #define X86_FEATURE_SMAP   (9 * 32 + 20)
9  
arch_handle_alternative(unsigned short feature,struct special_alt * alt)10  void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
11  {
12  	static struct special_alt *group, *prev;
13  
14  	/*
15  	 * Recompute orig_len for nested ALTERNATIVE()s.
16  	 */
17  	if (group && group->orig_sec == alt->orig_sec &&
18  	             group->orig_off == alt->orig_off) {
19  
20  		struct special_alt *iter = group;
21  		for (;;) {
22  			unsigned int len = max(iter->orig_len, alt->orig_len);
23  			iter->orig_len = alt->orig_len = len;
24  
25  			if (iter == prev)
26  				break;
27  
28  			iter = list_next_entry(iter, list);
29  		}
30  
31  	} else group = alt;
32  
33  	prev = alt;
34  
35  	switch (feature) {
36  	case X86_FEATURE_SMAP:
37  		/*
38  		 * If UACCESS validation is enabled; force that alternative;
39  		 * otherwise force it the other way.
40  		 *
41  		 * What we want to avoid is having both the original and the
42  		 * alternative code flow at the same time, in that case we can
43  		 * find paths that see the STAC but take the NOP instead of
44  		 * CLAC and the other way around.
45  		 */
46  		if (opts.uaccess)
47  			alt->skip_orig = true;
48  		else
49  			alt->skip_alt = true;
50  		break;
51  	case X86_FEATURE_POPCNT:
52  		/*
53  		 * It has been requested that we don't validate the !POPCNT
54  		 * feature path which is a "very very small percentage of
55  		 * machines".
56  		 */
57  		alt->skip_orig = true;
58  		break;
59  	default:
60  		break;
61  	}
62  }
63  
arch_support_alt_relocation(struct special_alt * special_alt,struct instruction * insn,struct reloc * reloc)64  bool arch_support_alt_relocation(struct special_alt *special_alt,
65  				 struct instruction *insn,
66  				 struct reloc *reloc)
67  {
68  	return true;
69  }
70  
71  /*
72   * There are 3 basic jump table patterns:
73   *
74   * 1. jmpq *[rodata addr](,%reg,8)
75   *
76   *    This is the most common case by far.  It jumps to an address in a simple
77   *    jump table which is stored in .rodata.
78   *
79   * 2. jmpq *[rodata addr](%rip)
80   *
81   *    This is caused by a rare GCC quirk, currently only seen in three driver
82   *    functions in the kernel, only with certain obscure non-distro configs.
83   *
84   *    As part of an optimization, GCC makes a copy of an existing switch jump
85   *    table, modifies it, and then hard-codes the jump (albeit with an indirect
86   *    jump) to use a single entry in the table.  The rest of the jump table and
87   *    some of its jump targets remain as dead code.
88   *
89   *    In such a case we can just crudely ignore all unreachable instruction
90   *    warnings for the entire object file.  Ideally we would just ignore them
91   *    for the function, but that would require redesigning the code quite a
92   *    bit.  And honestly that's just not worth doing: unreachable instruction
93   *    warnings are of questionable value anyway, and this is such a rare issue.
94   *
95   * 3. mov [rodata addr],%reg1
96   *    ... some instructions ...
97   *    jmpq *(%reg1,%reg2,8)
98   *
99   *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
100   *    writing, there are 11 occurrences of it in the allmodconfig kernel.
101   *
102   *    As of GCC 7 there are quite a few more of these and the 'in between' code
103   *    is significant. Esp. with KASAN enabled some of the code between the mov
104   *    and jmpq uses .rodata itself, which can confuse things.
105   *
106   *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
107   *    ensure the same register is used in the mov and jump instructions.
108   *
109   *    NOTE: MITIGATION_RETPOLINE made it harder still to decode dynamic jumps.
110   */
arch_find_switch_table(struct objtool_file * file,struct instruction * insn)111  struct reloc *arch_find_switch_table(struct objtool_file *file,
112  				    struct instruction *insn)
113  {
114  	struct reloc  *text_reloc, *rodata_reloc;
115  	struct section *table_sec;
116  	unsigned long table_offset;
117  
118  	/* look for a relocation which references .rodata */
119  	text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
120  					      insn->offset, insn->len);
121  	if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
122  	    !text_reloc->sym->sec->rodata)
123  		return NULL;
124  
125  	table_offset = reloc_addend(text_reloc);
126  	table_sec = text_reloc->sym->sec;
127  
128  	if (reloc_type(text_reloc) == R_X86_64_PC32)
129  		table_offset += 4;
130  
131  	/*
132  	 * Make sure the .rodata address isn't associated with a
133  	 * symbol.  GCC jump tables are anonymous data.
134  	 *
135  	 * Also support C jump tables which are in the same format as
136  	 * switch jump tables.  For objtool to recognize them, they
137  	 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
138  	 * have symbols associated with them.
139  	 */
140  	if (find_symbol_containing(table_sec, table_offset) &&
141  	    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
142  		return NULL;
143  
144  	/*
145  	 * Each table entry has a rela associated with it.  The rela
146  	 * should reference text in the same function as the original
147  	 * instruction.
148  	 */
149  	rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
150  	if (!rodata_reloc)
151  		return NULL;
152  
153  	/*
154  	 * Use of RIP-relative switch jumps is quite rare, and
155  	 * indicates a rare GCC quirk/bug which can leave dead
156  	 * code behind.
157  	 */
158  	if (reloc_type(text_reloc) == R_X86_64_PC32)
159  		file->ignore_unreachables = true;
160  
161  	return rodata_reloc;
162  }
163