[PATCH 3.13 154/198] arm64: ptrace: change fs when passing kernel pointer to regset code

From: Kamal Mostafa
Date: Tue Jul 15 2014 - 17:50:50 EST -stable review patch. If anyone has any objections, please let me know.


From: Will Deacon <will.deacon@xxxxxxx>

commit c168870704bcde6bb63d05f7882b620dd3985a46 upstream.

Our compat PTRACE_POKEUSR implementation simply passes the user data to
regset_copy_from_user after some simple range checking. Unfortunately,
the data in question has already been copied to the kernel stack by this
point, so the subsequent access_ok check fails and the ptrace request
returns -EFAULT. This causes problems tracing fork() with older versions
of strace.

This patch briefly changes the fs to KERNEL_DS, so that the access_ok
check passes even with a kernel address.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
arch/arm64/kernel/ptrace.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 6a8928b..bf288e4 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -821,6 +821,7 @@ static int compat_ptrace_write_user(struct task_struct *tsk, compat_ulong_t off,
compat_ulong_t val)
int ret;
+ mm_segment_t old_fs = get_fs();

if (off & 3 || off >= COMPAT_USER_SZ)
return -EIO;
@@ -828,10 +829,13 @@ static int compat_ptrace_write_user(struct task_struct *tsk, compat_ulong_t off,
if (off >= sizeof(compat_elf_gregset_t))
return 0;

+ set_fs(KERNEL_DS);
ret = copy_regset_from_user(tsk, &user_aarch32_view,
+ set_fs(old_fs);
return ret;


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