Linux: tracing: Keep the entry count when the histogram stats allocation fails

In the Linux kernel, the following vulnerability has been resolved:

tracing: Keep the entry count when the histogram stats allocation fails

print_entries() uses n_entries both as the number of sort entries and as its own return value, so the -ENOMEM it stores when the stats allocation fails overwrites the count that the cleanup still needs:

n_entries = tracing_map_sort_entries(map, ...); if (n_entries < 0) return n_entries; ... if (!stats) { n_entries = -ENOMEM; goto out; } ... out: tracing_map_destroy_sort_entries(sort_entries, n_entries);

tracing_map_destroy_sort_entries() takes an unsigned int and loops up to it, so -ENOMEM arrives as 4294967284. It walks an array of at most map->max_elts pointers and calls destroy_sort_entry(), which dereferences and frees, on whatever lies past the end.

Reading the hist file of a trigger with a .percent value, with that allocation forced to fail:

BUG: KASAN: vmalloc-out-of-bounds in tracing_map_destroy_sort_entries+0xa0/0xb0 Read of size 8 at addr ffffc90000045000 by task init/1 tracing_map_destroy_sort_entries+0xa0/0xb0 hist_show+0x6f7/0x1df0 seq_read_iter+0x2b8/0x1190 vfs_read+0x176/0xa40 The buggy address belongs to a 4-page vmalloc region starting at ffffc90000041000 allocated at tracing_map_sort_entries+0x5c/0xd50

A few pages further the fault is fatal. The registers at the oops confirm the bound: the loop's end pointer less the array start, over the pointer size, is 4294967284.

Return the error in a separate variable and leave n_entries holding the count, the way tracing_map_sort_entries() does on its own error path.

The stats block is only entered for a value carrying .percent or .graph, which __create_val_field() has rejected since v6.3, so this cannot be reached in mainline as it stands. It becomes reachable again with "tracing: hist: let values keep the percent and graph modifiers", so it should be applied first.