1// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
2// file at the top-level directory of this distribution and at
3// http://rust-lang.org/COPYRIGHT.
4//
5// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8// option. This file may not be copied, modified, or distributed
9// except according to those terms.
10
11//! This module defines the `__rust_probestack` intrinsic which is used in the
12//! implementation of "stack probes" on certain platforms.
13//!
14//! The purpose of a stack probe is to provide a static guarantee that if a
15//! thread has a guard page then a stack overflow is guaranteed to hit that
16//! guard page. If a function did not have a stack probe then there's a risk of
17//! having a stack frame *larger* than the guard page, so a function call could
18//! skip over the guard page entirely and then later hit maybe the heap or
19//! another thread, possibly leading to security vulnerabilities such as [The
20//! Stack Clash], for example.
21//!
22//! [The Stack Clash]: https://blog.qualys.com/securitylabs/2017/06/19/the-stack-clash
23//!
24//! The `__rust_probestack` is called in the prologue of functions whose stack
25//! size is larger than the guard page, for example larger than 4096 bytes on
26//! x86. This function is then responsible for "touching" all pages relevant to
27//! the stack to ensure that that if any of them are the guard page we'll hit
28//! them guaranteed.
29//!
30//! The precise ABI for how this function operates is defined by LLVM. There's
31//! no real documentation as to what this is, so you'd basically need to read
32//! the LLVM source code for reference. Often though the test cases can be
33//! illuminating as to the ABI that's generated, or just looking at the output
34//! of `llc`.
35//!
36//! Note that `#[naked]` is typically used here for the stack probe because the
37//! ABI corresponds to no actual ABI.
38//!
39//! Finally it's worth noting that at the time of this writing LLVM only has
40//! support for stack probes on x86 and x86_64. There's no support for stack
41//! probes on any other architecture like ARM or PowerPC64. LLVM I'm sure would
42//! be more than welcome to accept such a change!
43
44#![cfg(not(feature = "mangled-names"))]
45// Windows and Cygwin already has builtins to do this.
46#![cfg(not(any(windows, target_os = "cygwin")))]
47// All these builtins require assembly
48#![cfg(not(feature = "no-asm"))]
49// We only define stack probing for these architectures today.
50#![cfg(any(target_arch = "x86_64", target_arch = "x86"))]
51
52// Our goal here is to touch each page between %rsp+8 and %rsp+8-%rax,
53// ensuring that if any pages are unmapped we'll make a page fault.
54//
55// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
56// it does not actually match `extern "C"`.
57//
58// The ABI here is that the stack frame size is located in `%rax`. Upon
59// return we're not supposed to modify `%rsp` or `%rax`.
60#[cfg(target_arch = "x86_64")]
61#[unsafe(naked)]
62#[rustc_std_internal_symbol]
63pub unsafe extern "C" fn __rust_probestack() {
64 #[cfg(not(all(target_env = "sgx", target_vendor = "fortanix")))]
65 macro_rules! ret {
66 () => {
67 "ret"
68 };
69 }
70
71 #[cfg(all(target_env = "sgx", target_vendor = "fortanix"))]
72 macro_rules! ret {
73 // for this target, [manually patch for LVI].
74 //
75 // [manually patch for LVI]: https://software.intel.com/security-software-guidance/insights/deep-dive-load-value-injection#specialinstructions
76 () => {
77 "
78 pop %r11
79 lfence
80 jmp *%r11
81 "
82 };
83 }
84
85 core::arch::naked_asm!(
86 "
87 .cfi_startproc
88 pushq %rbp
89 .cfi_adjust_cfa_offset 8
90 .cfi_offset %rbp, -16
91 movq %rsp, %rbp
92 .cfi_def_cfa_register %rbp
93
94 mov %rax,%r11 // duplicate %rax as we're clobbering %r11
95
96 // Main loop, taken in one page increments. We're decrementing rsp by
97 // a page each time until there's less than a page remaining. We're
98 // guaranteed that this function isn't called unless there's more than a
99 // page needed.
100 //
101 // Note that we're also testing against `8(%rsp)` to account for the 8
102 // bytes pushed on the stack orginally with our return address. Using
103 // `8(%rsp)` simulates us testing the stack pointer in the caller's
104 // context.
105
106 // It's usually called when %rax >= 0x1000, but that's not always true.
107 // Dynamic stack allocation, which is needed to implement unsized
108 // rvalues, triggers stackprobe even if %rax < 0x1000.
109 // Thus we have to check %r11 first to avoid segfault.
110 cmp $0x1000,%r11
111 jna 3f
112 2:
113 sub $0x1000,%rsp
114 test %rsp,8(%rsp)
115 sub $0x1000,%r11
116 cmp $0x1000,%r11
117 ja 2b
118
119 3:
120 // Finish up the last remaining stack space requested, getting the last
121 // bits out of r11
122 sub %r11,%rsp
123 test %rsp,8(%rsp)
124
125 // Restore the stack pointer to what it previously was when entering
126 // this function. The caller will readjust the stack pointer after we
127 // return.
128 add %rax,%rsp
129
130 leave
131 .cfi_def_cfa_register %rsp
132 .cfi_adjust_cfa_offset -8
133 ",
134 ret!(),
135 "
136 .cfi_endproc
137 ",
138 options(att_syntax)
139 )
140}
141
142#[cfg(all(target_arch = "x86", not(target_os = "uefi")))]
143// This is the same as x86_64 above, only translated for 32-bit sizes. Note
144// that on Unix we're expected to restore everything as it was, this
145// function basically can't tamper with anything.
146//
147// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
148// it does not actually match `extern "C"`.
149//
150// The ABI here is the same as x86_64, except everything is 32-bits large.
151#[unsafe(naked)]
152#[rustc_std_internal_symbol]
153pub unsafe extern "C" fn __rust_probestack() {
154 core::arch::naked_asm!(
155 "
156 .cfi_startproc
157 push %ebp
158 .cfi_adjust_cfa_offset 4
159 .cfi_offset %ebp, -8
160 mov %esp, %ebp
161 .cfi_def_cfa_register %ebp
162 push %ecx
163 mov %eax,%ecx
164
165 cmp $0x1000,%ecx
166 jna 3f
167 2:
168 sub $0x1000,%esp
169 test %esp,8(%esp)
170 sub $0x1000,%ecx
171 cmp $0x1000,%ecx
172 ja 2b
173
174 3:
175 sub %ecx,%esp
176 test %esp,8(%esp)
177
178 add %eax,%esp
179 pop %ecx
180 leave
181 .cfi_def_cfa_register %esp
182 .cfi_adjust_cfa_offset -4
183 ret
184 .cfi_endproc
185 ",
186 options(att_syntax)
187 )
188}
189
190#[cfg(all(target_arch = "x86", target_os = "uefi"))]
191// UEFI target is windows like target. LLVM will do _chkstk things like windows.
192// probestack function will also do things like _chkstk in MSVC.
193// So we need to sub %ax %sp in probestack when arch is x86.
194//
195// FIXME(abi_custom): This function is unsafe because it uses a custom ABI,
196// it does not actually match `extern "C"`.
197//
198// REF: Rust commit(74e80468347)
199// rust\src\llvm-project\llvm\lib\Target\X86\X86FrameLowering.cpp: 805
200// Comments in LLVM:
201// MSVC x32's _chkstk and cygwin/mingw's _alloca adjust %esp themselves.
202// MSVC x64's __chkstk and cygwin/mingw's ___chkstk_ms do not adjust %rsp
203// themselves.
204#[unsafe(naked)]
205#[rustc_std_internal_symbol]
206pub unsafe extern "C" fn __rust_probestack() {
207 core::arch::naked_asm!(
208 "
209 .cfi_startproc
210 push %ebp
211 .cfi_adjust_cfa_offset 4
212 .cfi_offset %ebp, -8
213 mov %esp, %ebp
214 .cfi_def_cfa_register %ebp
215 push %ecx
216 push %edx
217 mov %eax,%ecx
218
219 cmp $0x1000,%ecx
220 jna 3f
221 2:
222 sub $0x1000,%esp
223 test %esp,8(%esp)
224 sub $0x1000,%ecx
225 cmp $0x1000,%ecx
226 ja 2b
227
228 3:
229 sub %ecx,%esp
230 test %esp,8(%esp)
231 mov 4(%ebp),%edx
232 mov %edx, 12(%esp)
233 add %eax,%esp
234 pop %edx
235 pop %ecx
236 leave
237
238 sub %eax, %esp
239 .cfi_def_cfa_register %esp
240 .cfi_adjust_cfa_offset -4
241 ret
242 .cfi_endproc
243 ",
244 options(att_syntax)
245 )
246}
247