Commit 96ae5227 authored by Sargun Dhillon's avatar Sargun Dhillon Committed by David S. Miller

bpf: Add bpf_probe_write_user BPF helper to be called in tracers

This allows user memory to be written to during the course of a kprobe.
It shouldn't be used to implement any kind of security mechanism
because of TOC-TOU attacks, but rather to debug, divert, and
manipulate execution of semi-cooperative processes.

Although it uses probe_kernel_write, we limit the address space
the probe can write into by checking the space with access_ok.
We do this as opposed to calling copy_to_user directly, in order
to avoid sleeping. In addition we ensure the threads's current fs
/ segment is USER_DS and the thread isn't exiting nor a kernel thread.

Given this feature is meant for experiments, and it has a risk of
crashing the system, and running programs, we print a warning on
when a proglet that attempts to use this helper is installed,
along with the pid and process name.
Signed-off-by: 's avatarSargun Dhillon <sargun@sargun.me>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: 's avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: 's avatarDavid S. Miller <davem@davemloft.net>
parent 9b022a6e
......@@ -365,6 +365,16 @@ enum bpf_func_id {
*/
BPF_FUNC_get_current_task,
/**
* bpf_probe_write_user(void *dst, void *src, int len)
* safely attempt to write to a location
* @dst: destination address in userspace
* @src: source address on stack
* @len: number of bytes to copy
* Return: 0 on success or negative error
*/
BPF_FUNC_probe_write_user,
__BPF_FUNC_MAX_ID,
};
......
......@@ -81,6 +81,49 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
.arg3_type = ARG_ANYTHING,
};
static u64 bpf_probe_write_user(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
{
void *unsafe_ptr = (void *) (long) r1;
void *src = (void *) (long) r2;
int size = (int) r3;
/*
* Ensure we're in user context which is safe for the helper to
* run. This helper has no business in a kthread.
*
* access_ok() should prevent writing to non-user memory, but in
* some situations (nommu, temporary switch, etc) access_ok() does
* not provide enough validation, hence the check on KERNEL_DS.
*/
if (unlikely(in_interrupt() ||
current->flags & (PF_KTHREAD | PF_EXITING)))
return -EPERM;
if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
return -EPERM;
if (!access_ok(VERIFY_WRITE, unsafe_ptr, size))
return -EPERM;
return probe_kernel_write(unsafe_ptr, src, size);
}
static const struct bpf_func_proto bpf_probe_write_user_proto = {
.func = bpf_probe_write_user,
.gpl_only = true,
.ret_type = RET_INTEGER,
.arg1_type = ARG_ANYTHING,
.arg2_type = ARG_PTR_TO_STACK,
.arg3_type = ARG_CONST_STACK_SIZE,
};
static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
{
pr_warn_ratelimited("%s[%d] is installing a program with bpf_probe_write_user helper that may corrupt user memory!",
current->comm, task_pid_nr(current));
return &bpf_probe_write_user_proto;
}
/*
* limited trace_printk()
* only %d %u %x %ld %lu %lx %lld %llu %llx %p %s conversion specifiers allowed
......@@ -362,6 +405,8 @@ static const struct bpf_func_proto *tracing_func_proto(enum bpf_func_id func_id)
return &bpf_get_smp_processor_id_proto;
case BPF_FUNC_perf_event_read:
return &bpf_perf_event_read_proto;
case BPF_FUNC_probe_write_user:
return bpf_get_probe_write_proto();
default:
return NULL;
}
......
......@@ -41,6 +41,8 @@ static int (*bpf_perf_event_output)(void *ctx, void *map, int index, void *data,
(void *) BPF_FUNC_perf_event_output;
static int (*bpf_get_stackid)(void *ctx, void *map, int flags) =
(void *) BPF_FUNC_get_stackid;
static int (*bpf_probe_write_user)(void *dst, void *src, int size) =
(void *) BPF_FUNC_probe_write_user;
/* llvm builtin functions that eBPF C program may use to
* emit BPF_LD_ABS and BPF_LD_IND instructions
......
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