x86-64: Preserve RCX and R11 when calling mprotect_asm (syscall) #673
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Test Output (Linux x86_64) |
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Very good catch, I'll make sure we merge this quick |
Grazfather
reviewed
Jul 6, 2021
On x86-64, the syscall instruction will clobber RCX and R11. It uses RCX to save the return address and uses R11 to save the RFLAGS. This means we will lose our old RCX and R11 value after the syscall returns to userspace. Bug reproduction steps (see how RCX and R11 change): $ gdb any-program # Run it inside the GDB starti # Set RCX and R11 before set-permission p $rcx=0 p $r11=0 # Do mprotect syscall set-permission $sp # Now the value of RCX and R11 have changed p $rcx p $r11 This commit fixes the bug by preserving RCX and R11. Signed-off-by: Ammar Faizi <[email protected]>
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Thanks for this! Good find. |
5 tasks
ammarfaizi2
added a commit
to ammarfaizi2/gef
that referenced
this pull request
Jul 7, 2021
This commit introduces 2 changes:
1) Change "info registers" to "info registers all". This will track more
registers and make sure they are not changed due to syscall.
2) Change `gdb_start_silent_cmd` to `gdb_run_cmd`. We don't need to use
`gdb_start_silent_cmd` because our `before` commands have already
started the process. And we can't see the register before we do
`set-permission` command if the process has not been started yet.
Therefore, it makes sense not to append `"entry-break"` (calling
`gdb_start_silent_cmd`).
Before this commit the result in commands will be like this:
before = [
# These two do the entry-break job!
"starti",
"si",
"printf \"match_before\\n\"",
"info registers all",
"printf \"match_before\\n\"",
"gef config context.clear_screen False",
"gef config context.layout '-code -stack'",
# This is unnecessary, because we have `starti` and `si`.
# We can't reorder it because it is appended inside the
# `gdb_start_silent_cmd`.
"entry-break"
]
cmd = "set-permission $sp"
after = [
"printf \"match_after\\n\"",
"info registers all",
"printf \"match_after\\n\""
]
After this commit:
before = [
"entry-break",
"printf \"match_before\\n\"",
"info registers all",
"printf \"match_before\\n\""
]
cmd = "set-permission $sp"
after = [
"printf \"match_after\\n\"",
"info registers all",
"printf \"match_after\\n\""
]
Link: hugsy#673 (comment)
Fixes: 5eb3b24 ("x86-64: Preserve RCX and R11 when calling mprotect_asm (syscall)")
Cc: Grazfather <[email protected]>
Signed-off-by: Ammar Faizi <[email protected]>
ammarfaizi2
added a commit
to ammarfaizi2/gef
that referenced
this pull request
Jul 7, 2021
This commit introduces 2 changes:
1) Change "info registers" to "info registers all". This will track more
registers and make sure they are not changed due to syscall.
2) Change `gdb_start_silent_cmd` to `gdb_run_cmd`. We don't need to use
`gdb_start_silent_cmd` because our `before` commands have already
started the process. And we can't see the register before we do
`set-permission` command if the process has not been started yet.
Therefore, it makes sense not to append `"entry-break"` (calling
`gdb_start_silent_cmd`).
Before this commit the result in commands will be like this:
before = [
# These two do the entry-break job!
"starti",
"si",
"printf \"match_before\\n\"",
"info registers all",
"printf \"match_before\\n\"",
"gef config context.clear_screen False",
"gef config context.layout '-code -stack'",
# This is unnecessary, because we have `starti` and `si`.
# We can't reorder it because it is appended inside the
# `gdb_start_silent_cmd`.
"entry-break"
]
cmd = "set-permission $sp"
after = [
"printf \"match_after\\n\"",
"info registers all",
"printf \"match_after\\n\""
]
After this commit:
before = [
"entry-break",
"printf \"match_before\\n\"",
"info registers all",
"printf \"match_before\\n\""
]
cmd = "set-permission $sp"
after = [
"printf \"match_after\\n\"",
"info registers all",
"printf \"match_after\\n\""
]
Link: hugsy#673 (comment)
Fixes: 5eb3b24 ("x86-64: Preserve RCX and R11 when calling mprotect_asm (syscall)")
Cc: Grazfather <[email protected]>
Signed-off-by: Ammar Faizi <[email protected]>
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x86-64: Preserve RCX and R11 when calling mprotect_asm (syscall)
How Has This Been Tested?
make testsChecklist
devbranch, notmaster.