powerpc/rtas: Fall back to linear search on failed token->function lookup

Enabling any of the powerpc:rtas_* tracepoints at boot is likely to
result in an oops on RTAS platforms. For example, booting a QEMU
pseries model with 'trace_event=powerpc:rtas_input' in the command
line leads to:

  BUG: Kernel NULL pointer dereference on read at 0x00000008
  Oops: Kernel access of bad area, sig: 7 [#1]
  NIP [c00000000004231c] do_enter_rtas+0x1bc/0x460
  LR [c00000000004231c] do_enter_rtas+0x1bc/0x460
  Call Trace:
    do_enter_rtas+0x1bc/0x460 (unreliable)
    rtas_call+0x22c/0x4a0
    rtas_get_boot_time+0x80/0x14c
    read_persistent_clock64+0x124/0x150
    read_persistent_wall_and_boot_offset+0x28/0x58
    timekeeping_init+0x70/0x348
    start_kernel+0xa0c/0xc1c
    start_here_common+0x1c/0x20

(This is preceded by a warning for the failed lookup in
rtas_token_to_function().)

This happens when __do_enter_rtas_trace() attempts a token to function
descriptor lookup before the xarray containing the mappings has been
set up.

Fall back to linear scan of the table if rtas_token_to_function_xarray
is empty.

Fixes: 24098f580e2b ("powerpc/rtas: add tracepoints around RTAS entry")
Reviewed-by: "Aneesh Kumar K.V (IBM)" <aneesh.kumar@kernel.org>
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-3-e9eafd0c8c6c@linux.ibm.com

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index ae9b10c954a1ba66a198c19a588f11c75dc448ec..f60a8e7bd5ed50a688de59fe5cd2c7c216e2d079 100644 (file)
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -572,11 +572,21 @@ static const struct rtas_function *rtas_token_to_function(s32 token)
                return NULL;
 
        func = rtas_token_to_function_untrusted(token);
+       if (func)
+               return func;
+       /*
+        * Fall back to linear scan in case the reverse mapping hasn't
+        * been initialized yet.
+        */
+       if (xa_empty(&rtas_token_to_function_xarray)) {
+               for_each_rtas_function(func) {
+                       if (func->token == token)
+                               return func;
+               }
+       }
 
-       if (WARN_ONCE(!func, "unexpected failed lookup for token %d", token))
-               return NULL;
-
-       return func;
+       WARN_ONCE(true, "unexpected failed lookup for token %d", token);
+       return NULL;
 }
 
 /* This is here deliberately so it's only used in this file */